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AVA 2001 - AVA Annual Meeting - Visual Adaptation 21 March 2001 AVA2001 Meeting theme: The Geoffrey J. Burton memorial lecture was given by: Professor Colin Blakemore FRS, Perceived Contrast Following Adaptation: The Role of Spatial Frequency & Stimulus Visibility Brendan T. Barrett, Paul V. McGraw, Paul Morrill Background/Aims: In the present experiments, we compare the reduction in perceived contrast following adaptation of test gratings with different orientations. Recent studies1 suggest that, under certain conditions, orthogonally oriented test gratings show a greater reduction in perceived contrast than those oriented parallel to the adapting stimulus. However, other investigators2,3 maintain that the reduction in perceived contrast is never greater than the parallel adapting and test condition. The aim of the present study was to resolve these conflicting findings. Methods: Using a contrast matching procedure we measured the perceived contrast of test gratings oriented at 45° and 135° following adaptation to a 45° grating of the same spatial frequency. Two experiments were carried out. Experiment 1: The perceived contrast of test gratings ranging in contrast from 1 to 0.45 was measured following adaptation to a grating with a contrast of 0.8. This was carried out at two spatial frequencies (2.25c/deg & 9c/deg). Experiment 2: The effects of adaptation upon perceived contrast were assessed for a number of adapting contrasts. A fixed ratio was maintained between the contrasts of the adapting and test gratings (1.33), and a range of spatial frequencies was investigated (2.25-9c/deg). Results: Experiment 1: The pattern of perceived contrast loss for the parallel and orthogonal conditions was very different for the high and low spatial frequency conditions. At the lower spatial frequency, the perceived contrast of the highest contrast test grating was reduced more for orthogonal adapting and test stimuli. This was not observed at the higher spatial frequency. Experiment 2: When the adapting stimulus was low in contrast, its effect was greatest upon parallel test gratings. However, when the adapting stimulus was high in contrast, its effect was greatest on test gratings orthogonal to the adapting stimulus. The contrast level above which orthogonal gratings showed more loss in perceived contrast than parallel gratings was found to increase with spatial frequency, suggesting that visibility of the adapting stimulus is the key to whether this effect is observed. Conclusions: Test gratings oriented orthogonally to an adapting grating can have a lower perceived contrast than parallel gratings of the same physical contrast provided the visibility of the adapting stimulus is high enough. References:
Increased visual after-effects following pattern adaptation in migraine. Alex Shepherd, Much previous research into visual processing in migraine has examined low-level aspects of visual processing, often using detection or discrimination measures and the very stimuli reported to trigger an attack, such as striped patterns or flickering lights. Differences between people with and without migraine have been attributed to abnormal cortical processing in migraine, variously described in terms of interictal hyperexcitability, heightened responsiveness, a lack of habituation and/or a lack of intra-cortical inhibition. Here two experiments are presented that explore a uniquely cortical phenomenon, pattern or contrast adaptation. Pattern adaptation reflects mutual interactions between groups of neurones and is therefore ideally suited to address proposed models of cortical function in migraine. These models lead to specific predictions in an adaptation study: there should be smaller effects in people with migraine than in people without. Two adaptation experiments, one using the motion after-effect, one the tilt after-effect, however, both revealed larger effects in migraine sufferers than in headache-free control subjects. These results will be discussed in terms of models of cortical function in migraine.
BODMAS: An account of contrast adaptation on contrast perception Keith Langley, Background Snowden and Hammett (1992) proposed that both summative and divisive mechanisms may account for effects of contrast adaptation on contrast perception. Ross and Speed (1996), however, rejected these ideas, proposing instead that adaptation may be described by changes in the exponent and the semi-saturation constant of the Naka-Rushton receptor equation. To help resolve the debate, the parametric effects of contrast adaptation on contrast perception are examined. Methods A 2AFC paradigm was used to match the contrast matching spatially adjacent sinusoidal gratings of equal spatial frequency. The adapting pattern was fixed at 88% contrast, while both the contrast and orientation of the test pattern was varied. Results Effects of contrast adaptation on contrast perception showed atypical orientation tuning when the test pattern was presented near threshold. For higher test patterns, however, effects of contrast adaptation on contrast perception did not show orientation tuning, but rather a near isotropic suppression of perceived contrast. To interpret the results, a Naka-Rushton receptor equation that allows for the possibility of changes in exponent, semi-saturation constant, and summative offset was fitted to the contrast matching function. Conclusions The model fits suggest, when adapting at high contrast, and testing across a range of different contrasts and orientations, that effects of adaptation may be summarized by BODMAS (Brackets, Order, Division, Multiplication, Addition and Subtraction) as a parameterization of effects of contrast adaptation on contrast perception. This parameterization may be simplified further, if it is assumed that the contrast matching function converges with the line of no-effect at 100% contrast. The latter assumption reveals both a summative (a directionally specific subtractive and an isotropic additive) component and an isotropic amplification of contrast signals because the divisive element counterbalances perceived contrast increases owing to signal amplification. The implications of the results with regard to contrast discrimination are discussed. Acknowledgement: Some of this research was conducted with the assistance of G. Abbonizio.
Using adaptation to probe the sources of cross-orientation suppression in the primary visual cortex Tobe C B Freeman, Severine Durand, Daniel C Kiper and Matteo Carandini We have exploited the phenomenon of visual adaptation to explore the interactions between neurons tuned to different orientations in the primary visual cortex (V1). Neurons in V1 give little response to bars that are orthogonal to their preferred orientation. Such a stimulus, however, can substantially suppress the neuron's responses to optimally oriented bars. This "cross-orientation suppression" is commonly thought to originate from a pool of cortical neurons through intracortical inhibition. We reasoned that if suppression originates from intracortical connections, it should be possible to reduce its effectiveness through selective adaptation. Prolonged presentation of the orthogonal stimulus would strongly reduce (adapt) the responses of those cortical neurons that are selective for it, and thus reduce the suppression that these neurons can provide. We recorded from neurons in the primary visual cortex of anesthetized and paralyzed cats. Visual stimuli consisted of a drifting grating at the cell's preferred orientation (test) combined with a second orthogonal drifting grating (mask). Responses were measured following adaptation to sustained presentation (30 sec initially, with 4-6 sec 'top-ups' prior to each stimulus presentation) of the test, the mask, or a blank screen. Responses were fitted with a simple divisive model of cross-orientation suppression that depends on test contrast and mask contrast. We tested the prediction that adaptation would diminish cross-orientation suppression and found it to be incorrect: Intriguingly, cross-orientation suppression is immune to adaptation. Because primary visual cortex neurons that do not adapt are observed rarely, our results cast doubt on the view that cross-orientation suppression originates from cortical neurons. We speculate that it may result from depression at thalamocortical synapses. Supported by Swiss National Science Foundation and Human Frontiers Science Program.
Shading and texture: separate information channels with a common adaptation mechanism? Mark Georgeson & Andrew Schofield, We outline a scheme for the way in which vision may handle information about shading (luminance modulation, LM) and texture (contrast modulation, CM). This scheme aims to reconcile a variety of findings - from experiments on adaptation to LM and CM and on detection and discrimination of LM and CM patterns - that otherwise sit uncomfortably together. LM and CM gratings (presented in 2-D static spatial noise carriers) appear to be detected independently, and there is no sub-threshold summation between LM and CM. This strongly suggests separate channels for the detection of LM and CM structure (Schofield & Georgeson, Vision Research 1999). Adapting to luminance gratings selectively lowers the detectability of luminance gratings but not CM gratings, and vice-versa (Nishida et al, Vision Research 1997), again suggesting separate channels. However, we now report experiments in which adapting to LM (or CM) gratings creates tilt aftereffects of similar magnitude on both LM and CM test gratings, and reduces the perceived strength (modulation depth) of LM and CM gratings to a similar extent. This might suggest a second stage of processing at which LM and CM information is pooled, and such second-stage integration has been a feature of most models for second-order motion processing. The nature of this integration, however, is unclear and several simple predictions are not fulfilled. Firstly, one might expect the integration stage to signal the spatial structure (orientation, spatial frequency, modulation depth) of both LM and CM patterns but to lose identity information: was the pattern LM or CM ? We show instead that sensitivities for detection and for identification of LM and CM patterns are about the same. Thus the identity of even very weak LM and CM patterns is not lost. Secondly, when LM and CM gratings are combined out-of-phase there is no cancellation. Detectability of in-phase and out-of-phase pairings is very similar, and identification of the phase relation is as good as the ability to identify single LM or CM components. These results suggest that information about LM and CM is not pooled or merged. Shading is not confused with texture variation. We conclude that LM and CM signals are carried by separate channels, perhaps because, over an ensemble of images, the first and second-order content of natural images is uncorrelated (Schofield, Perception, 2000). These channels share a common adaptation mechanism that accounts for the complete transfer of perceptual aftereffects. The fact that threshold elevation is selective and does not transfer between LM and CM suggests a second, perhaps earlier, site of adaptation within each separate channel.
Evidence for spatio-temporal selectivity in attentional modulation of the motion aftereffect JP Harris, MS Georgiades Department of Psychology, When attention is paid to one region of the visual field, and withdrawn from another, the ignored region still needs to be monitored for potentially interesting events. This might be done by an intermittent full visual analysis or by a more continuous but restricted analysis. We investigated which type of process is more likely in early vision by studying the effects of diverting attention on adaptation to a range of spatial (0.5, 2, 4, and 6 c/deg) and temporal (1.5 and 10 Hz) frequencies. During adaptation, subjects either fixated an unchanging digit (normal attention), or named the sequence of changing digits which formed the fixation point (diverted). The test field was always a static version of the adapting field, and the strength of adaptation was measured through the duration of subsequent Motion Aftereffects (MAEs). When attention during adaptation was normal, MAE durations rose with spatial frequency for the 1.5 Hz stimuli, and declined with spatial frequency for the 10 Hz stimuli. When attention was diverted from the 10 Hz stimuli, MAE durations fell by a similar amount at all spatial frequencies. However, for the 1.5 Hz stimuli, the effects of diversion were very small at 0.5 c/deg, and rose progressively with spatial frequency, so that MAE reductions were largest at 6 c/deg. It appears that diversion hardly affects the encoding of coarse, slow, stimuli, but attenuates the encoding of finer/and or faster stimuli. This is consistent with the idea that during diversion the visual system monitors the scene continuously, but over a restricted range of spatial and temporal scales.
Understanding motion adaptation: P Thompson [1], S T Hammett [2], S Bedington[2] ([1] Psychology, University of York, [2]Psychology, Royal Holloway, University of London Motion perception involves both speed and direction, whilst research into motion perception lacks both. Many psychophysical studies have exploited adaptation to probe the mechanisms underlying direction and speed coding. Unfortunately we have a very poor understanding of the physiological underpinnings of adaptation and even less idea about how speed is encoded in the visual system. We can reject the most simple-minded model (Barlow and Hill 1963) that speed is encoded directly in firing rate and that adaptation is a matter of fatigue. However we have found support for the second most simple-minded model. We have examined the perceived speed of gratings before and after adaptation. using a yes/no matching task informing a PEST routine. Both the build-up of adaptation over time and the recovery from adaptation were examined. The results show: (i) perceived speed declines exponentially with adaptation duration; (ii) the time constants for 2Hz and 12Hz adaptation rates are different (15.88 and 1.9 seconds respectively). We show that a model that assumes that speed is based upon the ratio of two underlying mechanisms (with time constants of 2-4 seconds) can capture much of these data. We can conclude very little about the nature of adaptation from these results but a plausible model of speed processing can be fabricated from them.
Changes in the temporal integration due to adaptation. Robert J. Snowden ,School of Psychology, Cardiff University During adaptation to decreasing luminance level it is well documented that the response of the visual system changes so as to increase the time over which information about luminance is integrated. It therefore seems plausible that during adaptation to contrast the visual system might also change the amount of time over which information about contrast is extracted. To test this notion temporal integration functions were measured in two ways. Firstly contrast sensitivity was measured as a function of stimulus duration for tests presented on a blank screen or upon a pedestal stimulus. Likewise two-pulse summation thresholds were measured (for a range of pulse intervals) in similar circumstances. Both experiments show that information is gathered over a shorter time period when presented on a pedestal stimulus. These results suggest the visual system alters its temporal characteristics in response to the presentation of a high contrast pattern.
Obligatory averaging of orientation in crowded stimuli M. Morgan and J. Solomon , AVRC, City University, London Classification of a target object in peripheral vision can be impaired when distractor objects appear close to it, even when the observer knows which object is the target. To determine the cause of this phenomenon (“crowding”), we asked human observers to report the orientation (clockwise or anti-clockwise) of one or more tilted grating patches (the targets) when presented amongst horizontal distractors. Threshold tilts increased linearly with the number of distractors and decreased with the number of targets. This suggests that the estimated orientation of the target is pooled with those of the distractors. An early noise, which corrupts local estimates of orientation and a late noise, which corrupts their average, must be invoked to explain the results. Obligatory averaging was further supported by the finding that distractors with a slight tilt in the direction of the target do not impair performance as much as horizontal distractors, which in turn do not impair performance as much as distractors with a slight tilt in the direction opposite that of the target. We conclude that local estimates of orientation are not degraded by crowding, but observers have no conscious access to these estimates and must rely on an average signal.
Within and between cue-adaptation using motion parallax, stereopsis and luminance. M.F. Bradshaw, P.B. Hibbard & M.A. Hogervorst The processing of luminance, motion parallax and stereopsis appears tuned to particular spatial or corrugation frequencies. Here we investigate the nature of such processing further by determining whether perceived frequency (spatial frequency shift AE) and perceived orientation (tilt AE) can be altered by adaptation to stimuli defined by motion parallax, binocular disparity or luminance. Both phenomena exist in the luminance and stereopsis domains (Blakemore and Sutton, 1969, Science 166, 245-247; Tyler, 1975, Perception 4, 187-192) but have yet to be established in the parallax domain. We also address whether adaptation in one domain (e.g. luminance) affects the perceived size or orientation defined in another domain (e.g. parallax). Observers adapted to two patterns (5 deg in diameter) arranged in a horizontal 'dumbbell' configuration. The 1 cpd adapting patterns differed in either orientation (± 15, ±30, ±45 or ±60 degrees) or in spatial frequency (±0.125, ±0.25, ± 0.5, ±0.75 octaves). A staircase procedure was employed to determine when the test surfaces (left and right parts of the dumbell) appeared to be of equal orientation or spatial frequency. The initial period of adaptation was 120 sec followed by a test-adaptation cycle of 2 sec test and 4 sec adaptation. The phase of the adapting patterns changed every 2 secs to avoid phase dependent aftereffects. Both size and orientation aftereffects were found within the motion parallax domain. The peak magnitude of the aftereffect (~ 3 degrees), the location of its peak (30 degrees) and its estimated bandwidth (30 degrees) were similar to those already established, and replicated here, in the cyclopean domain (Tyler 1975, op cit). The peak magnitude of the size aftereffect was ~12% in all three domains and occurred with adapting patterns which differed by 0.75 octaves. Cross adaptation of size and orientation was possible ( 75% of the within cue effect) between parallax, stereo and luminance defined contours which suggests a common stage in both size and orientation processing.
Orientation Mechanisms from simple cell phase Tim Atherton The notions of frequency- and orientation-selective filtering adequately describe the “simple” cell stage of processing in visual cortex. Models of the “second stage” mechanisms are less clear. Second stage orientation mechanisms have been proposed that use the “energy” responses of simple cells (Langley & Atherton 1991). These mechanisms might account for some forms of orientation pop-out, they give the correct orientation for a sinusoidal grating image, but as a uniform field, failing completely to account for the percept of light and dark bars. Many authors have suggested that phase plays an important role in visual processing, see for example (Burr & Morrone 1990; Georgeson & Freeman 1997; Morrone & Burr 1988). The model we propose extends that of Langley, (Langley & Atherton 1991), to process the energy, real, and imaginary responses of simple cells in three parallel streams. The model accurately estimates a family of orientations and, for example, places the "edges" and centres of light and dark bars of a grating in good agreement with the positions perceived by human subjects. The model explains the checkerboard appearance of “plaids” composed of two or three gratings, provides mechanisms for orientation pop-out, and it reproduces the Mach band effect. The model goes beyond simple “edge” and “bar” features to detect and quantify the orientation of local patterns with higher-order rotational symmetries. The processing implied by the proposed model is biologically plausible, surprisingly simple, robust, and unifies the processing of energy and phase mechanisms. The second-stage processing results in a family of feature maps that have implications for the understanding of later processing eg pop-out and texture segmentation. The nature of the proposed processing suggests “complex” cells with a variety of properties. The talk will detail the model and be illustrated by numerous examples of the analysis of test and real-world images, with indications of the expected receptive field properties of some of the second stage mechanisms. Burr, D. C. & Morrone, M. C. 1990 Feature Detection in Biological and Artificial Visual Systems. In Vision: Coding and Efficiency (ed. C. Blakemore), pp. 185-194. Cambridge: Cambridge University Press. Georgeson, M. A. & Freeman, T. C. A. 1997 Perceived Location of Bars and Edges in One-dimensional Images: Computational Models and Human Vision. Vision Research 37, 127-142. Langley, K. & Atherton, T. J. 1991 The Inference of Structure in Images using Multi-Local Quadrature Filters. In British Machine Vision Conference 1991 (ed. P. Mowforth), pp. 111-117. Glasgow: Springer-Verlag. Morrone, M. C. & Burr, D. C. 1988 Feature Detection in Human Vision: a Phase-dependent Energy Model. Proc. R. Soc. Lond. B 235, 221-245.
Changes in bias and sensitivity of directional motion judgements in the flicker motion after effect (FMAE) and velocity aftereffect (VAE). Michael J.Wright and Louise Alston Adaptation to moving stimuli changes the perceived velocity of test stimuli, but does adaptation alter differential sensitivity to velocity? Flicker motion aftereffects (FMAE) and velocity aftereffects (VAE) were measured with a single-interval forced choice method, using directionally-ambiguous test stimuli (Ledgeway, T. (1994) Vision Research, 34, 2879-2889; Wright, M.J. (1998), Perception, 27, 1489). The test stimulus for FMAE was a counterphase grating, generated by superimposing two opposite-motion drifting gratings. The test stimulus for VAE consisted of the same two components spatially separated. Either the relative velocity or the relative contrast of the two components was varied, to produce a set of constant stimuli. The task was to indicate the stronger overall direction of motion in the test stimulus (upwards or downwards). In the unadapted state, motion was seen to predominate in the direction of the faster or the higher contrast component. 9 pairs of contrast values or velocity values were repeated 12 times each in a single run. This generated a psychometric function, fitted by probit analysis, from which sensitivity and bias measures could be obtained. After adaptation to a drifting grating or plaid, motion was biased strongly in the opposite direction. The tuning curves of bias versus grating velocity differed for FMAE and VAE. The tuning of FMAE to the orientation of plaid components was found to be broad, and it was related to the velocity of grating components, rather than plaid pattern velocity. It was found that for both FMAE and VAE there are changes in sensitivity as well as bias. Over a small range, the motion system appears to maintain differential directional sensitivity to motion despite a shift in bias, but with adaptation sufficient to produce strong aftereffects, directional sensitivity is reduced.
Pulling the envelope: the influence of motion adaptation on perceived position P. V. McGraw [1], S. Chung [2], D. Whitaker [1], J. Skillen [1] Following adaptation to a moving stimulus, texture within a stationary stimulus is perceived to drift in the opposite direction - the traditional motion after-effect (MAE). Until recently, it was thought that the MAE did not produce a concurrent change in object position. However, it has recently been shown that perceived position can be markedly influenced by motion signals (1). In the present study we ask whether position can be modified in the absence of perceived motion. The stimulus elements (Gabor patches) consisted of Gaussian-windowed (envelope) sinusoidal luminance modulations (carrier), presented in a 2-blob vernier alignment test. Prior to the test phase, subjects adapted to Gabor stimuli in which the carrier gratings drifted in opposite directions. The elements of the adapting stimulus were spatially coincident with the test elements in the 2-blob alignment task. Following adaptation an illusory misalignment of the elements of the test stimulus was perceived, and the magnitude of this perceived offset was established. When the adapting and test stimuli have collinear carrier gratings, the global position of the object shows a substantial shift in the direction of the illusory motion. When the carrier gratings of the adapting and test stimuli are orthogonal (a configuration where no MAE is experienced) a global positional shift of similar magnitude is found. The illusory positional shift was also found to be immune to changes in spatial frequency and contrast. The lack of sensitivity for stimulus characteristics other than direction of motion suggests that the mechanism which produces the shift in spatial position is distinct from that which mediates the traditional MAE.
Contrast masking, adaptation and gain pool summation (T. S. Meese & D. J. Holmes; Neurosciences Research Institute, Vision Sciences, Aston University, Birmingham B47ET, UK.; Foley (1994; Journal of the Optical Society of America A, 11, 1710-1719) has proposed an influential psychophysical model of masking in which mask components in a contrast gain pool are raised to an exponent before summation and divisive inhibition. We tested this summation rule in experiments in which contrast detection thresholds were measured for a vertical 1 c/deg (or 2 c/deg) sine-wave component in the presence of a 3 c/deg (or 6 c/deg) mask that had either a single component oriented at -45° or a pair of components oriented at ±45°. Contrary to the predictions of Foley's model 3, we found that for masks of moderate contrast and above, threshold elevation was predicted by linear summation of the mask components in the inhibitory stage of the contrast gain pool. We built this feature into two new models: the early adaptation model and the hybrid model. In the early adaptation model, contrast adaptation controls a threshold-like nonlinearity on the output of quasi-linear pathways that provide the excitatory and inhibitory inputs to a gain control stage. This stage also receives inhibitory input from a constant and a weighted pool of mask pathways that are summed before being raised to an exponent. The hybrid model is a hybrid of Foley's (1994) models 2 & 3 and has been modified to handle adaptation. Specifically, it includes non-adaptable pathways with expansive nonlinearities that provide excitatory and inhibitory inputs to a gain control stage. Adaptation of the excitatory pathway controls the combined inhibitory weight of (1) a semisaturation constant and (2) the linear sum of all other pathways in a gain pool raised to an expansive component. With only six free parameters, both models provide excellent fits to the masking and adaptation data of Foley and Chen (1997; Vision Research, 37, 2779-2788), where contrast detection was performed in the presence of parallel and orthogonal masks after adaptation to either vertical-, horizontal-, plaid- or no-Gabor stimuli. Furthermore, unlike Foley and Chen's 'two process' adaptation version of Foley's model 3, only one parameter was needed to control the state of adaptation. Only the hybrid model was able to capture the features of an experiment in which contrast increment thresholds were measured in the presence of an orthogonal mask (Foley, 1994). Only the early adaptation model was able to capture a finding reported here, that masking by a grating is slightly greater than for a plaid at low mask contrast. We conclude that (1) linear summation of inhibitory components is a feature of contrast masking, and (2) that the main aftereffect of spatial adaptation on contrast increment thresholds can be assigned to a single site.
Contextual masking in marginal vision: role for motor training purposes ABSTRACT: When focusing to a visual object, peripheral stimuli must be monitored for appropriate control of attention at the surrounding environment. Thresholds for shape recognition of a peripheral stimulus were studied in human subjects with stimuli back-projected on a screen at a viewing distance of 220 cm. First of all object threshold perception for each person was determined, and used as starting point for the subsequent study. Thresholds in nasal retina are lower than for temporal retina. Results show that both in nasal and temporal 60° eccentricity retinal fields, most subjects correctly recognize shapes when surrounded by confounding similar forms, recognition of different shapes are sometimes confounded. We are in agreement with the full paper request for publishing in the journal. F. Huertas, J. M. Castellote, M.T.Sanegre
Can we learn to adapt to enhanced binocular disparities ? M.F. Bradshaw, P.B. Hibbard N. Stringer, SJ. Watt and A.R. Willis. If the inter-ocular distance (IOD) is increased beyond normal, the magnitude of the disparity information is magnified. We have shown that such a manipulation improves performance in nulling and matching tasks. Here we examine whether observers can learn to use enhanced disparities to accurately perform tasks requiring the recovery of Euclidean geometry (a shape task). Inter-ocular base was adjusted by simulation within a stereoscopic display. The design comprised three phases: pre-adaptation (IOD = 6.5 cm), adaptation (IOD = 3.25 or 13 cm) and post- adaptation (IOD = 6.5 cm). Observers were required to adjust the magnitude of a depth interval (specified by binocular disparity) so that it matched a specified 2D interval specified by two lights (set between 5 and 15 cm) in an otherwise blacked-out scene. In the adaptation phase, the observers adjusted the depth interval repeatedly until a performance criterion was reached. Two forms of feedback were given in the adaptation phase: direct, where another light was shown at the correct disparity; and symbolic, where a signed number indicated the magnitude and direction of the error. Observers were clearly affected by the change in IOD but learned the new IOD rapidly under both feedback conditions.
Luminance adaptation and perceptual filling-in. Andrew E Welchman & Julie M Harris A grey target peripherally presented within dynamic random noise (DRN) perceptually fades from view (or “fills-in”) following a period of steady viewing. The observer’s perception is then one of DRN at the target’s location. To gain insight into the neural mechanisms responsible for this perception we investigated whether luminance adaptation occurs in the presence of perceptual filling-in. Observers viewed a computer screen filled with DRN. They were instructed to detect the presence of a small, bright test probe presented at the target’s centre (eccentricity = 10 deg). Luminance increment thresholds (the amount of extra luminance needed to detect the probe) were obtained for 2 conditions: (1) No adaptation – test intervals were presented after trial initiation; (2) Adaptation – observers viewed the stimulus until they reported target disappearance, and then viewed the test intervals. Detection thresholds were obtained for a range of target luminance values. Lluminance increment thresholds were lower with adaptation (condition 2) than when observers simply viewed the test (condition 1). In other words, it was easier for subjects to detect the probe when they could not perceive the target, than when they could. This result could indicate gain control in the neurones that encode the presence of the target. Alternatively, it may be related to transient detection and not related to perceptual filling-in per se.
On the latency of visual perception: the role of display parameters David Rose Features are not all processed at the same speed, and our awareness is not necessarily correlated with the latency of processing. This raises problems for how we perceive and act accurately in a world of moving objects. The recent attempts by Zeki, Moutoussis and Bartels to measure the relative latencies of awareness have led to estimates of asynchronies of the order of 100 msec between, for example, colour and motion awarenesses. However, their displays were complex, and simpler stimuli have been found not to engender any such asynchrony (Nishida and Johnston, ARVO 1999, 2000, ECVP 2000; Rose, ASSC 1999, ECVP 1999, Foursites 1999). Here, I investigate the display characteristics which may induce asynchronies. Moving dots changed colour and changed direction of motion at various relative times, and subjects identified which changed first, in a 2AFC task. Surprisingly, asynchronies only occurred when two conditions were simultaneously present: repetitive changes in the display (rather then single changes), and requiring subjects to judge where colour changed relative to the mid-points (rather than the end-points) of the motion trajectory. Neither condition on its own induced asynchrony (which was also not observed with both conditions absent). Various possible artefacts in displays such as these are discussed. The results pose further problems for theories of perceptual latency.
Perception and action are affected by 2-D tilt and 3-D slant illusions Simon J. Watt, Mark F. Bradshaw and Paul B. Hibbard Neuropsychological evidence suggests a strong dissociation between visual processing for perception and for the control of action (Milner and Goodale, 1995). Although equivocal, the major source of evidence in normals is that actions such as grasping may be less susceptible than perceptual responses to the influence of common visual illusions (Aglioti et al., 1995; Franz et al, 2000). We investigated the relative effects on perception- and action-based responses of 2-D tilt and 3-D slant illusions, using “posting” tasks similar to those employed originally by Milner and Goodale. In the 2-D tilt contrast condition subjects were required either to post a card through, or set a paddle to match the orientation of, a vertical plane which was surrounded by a striped surface tilted between ± 60 degs. In the 3-D slant contrast condition, subjects performed similar tasks, but the vertical plane was surrounded by a disparity defined surface slanted or inclined in depth between ± 60 degs. Both the 2-D tilt illusion and the 3-D slant illusion influenced perception- and action- based responses. If anything, the magnitude of the effect of the illusions was greater for the posting task than for the matching task. We conclude that no qualitative differences exist between perception- and action-based tasks for the 2-D tilt illusion and the 3-D slant contrast illusion.
The effect of temporal delay on the ability to control prehensile movements in 5 to 11 year old children. Kathleen M. Elliott1, M.F. Bradshaw1, S.J. Watt1 and P.M. Riddell2 Neuropsychological evidence suggests a dissociation between the visual systems involved in perceptual processes and those involved in the control of action (Milner and Goodale, 1995). According to this account information for the control of action needs to be updated rapidly, as object properties such as egocentric distance change continuously as an observer moves around the world. Therefore this system may only retain information for a brief period. Consistent with this, it has been reported that reaching and grasping performance in adults deteriorates when a delay is introduced between target presentation and response (Bradshaw et al., 1998; Hu et al., 1999; Watt and Bradshaw, 2000). Here we investigate whether temporal delay similarly affects the control of prehensile movements in middle childhood. Twenty five children aged 5-11 years made open-loop reaches to 2 different sized objects placed at 15 and 25 cm along the midline. We compared reaches where subjects' vision was occluded at movement onset (using LCD goggles) and where there was a 2 second delay between occlusion of subjects' vision and their response. A MacReflex motion analysis system was used to record kinematic and spatial parameters of the movements. Temporal delay affected principal indices of both the transport and grasp components of children's reaches. Compared to the no-delay condition, peak wrist velocity was slower and grip apertures were wider following a 2 second delay. Delay also resulted in a longer time spent in the final slow movement phase of the reach. In a further analysis the subjects were divided into three age groups to examine whether the effects of delay varied with age. However, no differences were found across age groups. We conclude that visual information for the control of prehension in children deteriorates rapidly, and in a similar fashion to that reported previously for adults. |
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