Peter Wenderoth

A functional angle on some after-effects in cortical vision

The question of how our brains and those of other animals code sensory information is of fundamental importance to neuroscience research. Visual illusions offer valuable insight into the mechanisms of perceptual coding. One such illusion, the tilt after–effect (TAE), has been studied extensively since the 1930s, yet a full explanation of the effect has remained elusive. Here, we put forward an explanation of the TAE in terms of a functional role for adaptation in the visual cortex. The proposed model accounts not only for the phenomenology of the TAE, but also for spatial interactions in perceived tilt and the effects of adaptation on the perception of direction of motion and colour. We discuss the implications of the model for understanding the effects of adaptation and surround stimulation on the response properties of cortical neurons.

The salience of vertical symmetry

It has long been accepted that amongst patterns which are bilaterally symmetrical, those which have their axis of symmetry vertical are more saliently symmetrical than patterns whose axis of symmetry is at some other orientation. The evidence regarding the relative salience of other orientations of axis of symmetry is somewhat more equivocal. In experiment 1, subjects were required to discriminate between symmetric or random-dot patterns when the axis of symmetry was at one of eighteen different orientations, spaced 10° apart, both clockwise and counterclockwise of vertical to horizontal. The data indicated that vertical was most salient, then horizontal but that, unlike in the classical oblique effect for contrast sensitivity, performance for precisely diagonal axes was better than that for surrounding axis orientations. Additional data (from experiments 2 and 3) also showed that the salience of vertical and horizontal axes of symmetry can be manipulated extensively by varying the range of stimuli presented, presumably by manipulating the scanning or attentional strategy adopted by the observer. Many previous studies of symmetry perception may have confounded hard-wired salience for vertical symmetry with scanning or attentional strategies.

The different mechanisms of the direct and indirect tilt illusions

Both the tilt illusion and aftereffect exhibit indirect effects under certain conditions: these are negative (assimilation) effects which occur with large (70-90 deg) angular separations between test and inducing gratings. They are opposite in direction to the positive, and much larger, contrast effects which occur at smaller (10-15 deg) separations. Evidence from six experiments shows that stimulus manipulations which reduce direct effects have little or no effect on indirect effects and vice versa, suggesting that the two effects have different determinants. It is proposed that direct effects arise from lateral inhibitory interactions between populations of neurones in striate cortex and that indirect effects occur at a higher level, possibly in areas concerned with stimulus-specific interactions beyond the classic receptive field. The implications of the data for theories of the tilt illusion are considered.

The tilt illusion: Repulsion and attraction effects in the oblique meridian

Abstract It is commonly believed that the same neural mechanism underlies the tilt illusion and the tilt aftereffect. Recently, it has been demonstrated that tilt aftereffects induced on oblique stimuli are similar, in magnitude and direction, to those induced on stimuli which are vertical or horizontal. If the mechanisms of the illusion and aftereffect are the same, then illusions induced on oblique stimuli should also be similar to those induced on vertical or horizontal stimuli. The six experiments reported here confirmed this prediction by suggesting that both repulsion (direct) and attraction (indirect) tilt illusions occur in the oblique meridian. The data are considered in relation to both psychophysical (normalisation) and neural (lateral inhibition) theories of orientation illusions and aftereffects.

The influence of colour and contour rivalry on the magnitude of the tilt after-effect.

Abstract Tilt after-effects were generated by inspection of gratings inclined 10 or 15° from the vertical in six experiments. The results indicated that the magnitude of the tilt after-effect: was not influenced by the colour of the inspection and test gratings (Experiments I–IV); was not affected by binocular rivalry suppression (Experiment V); and was the same under various conditions of monocular and binocular inspection and testing (Experiment VI).

Asynchronous processing in vision: color leads motion.

It has been demonstrated that subjects do not report changes in color and direction of motion as being co-incidental when they occur synchronously. Instead, for the changes to be reported as being synchronous, changes in direction of motion must precede changes in color. To explain this observation, some researchers have suggested that the neural processing of color and motion is asynchronous. This interpretation has been criticized on the basis that processing time may not correlate directly and invariantly with perceived time of occurrence. Here we examine this possibility by making use of the color-contingent motion aftereffect. By correlating color states disproportionately with two directions of motion, we produced and measured color-contingent motion aftereffects as a function of the range of physical correlations. The aftereffects observed are consistent with the perceptual correlation between color and motion being different from the physical correlation. These findings demonstrate asynchronous processing for different stimulus attributes, with color being processed more quickly than motion. This suggests that the time course of perceptual experience correlates directly with that of neural activity.

Orthogonal adaptation improves orientation discrimination

We investigated the effect of adaptation on orientation discrimination using two experienced observers, then replicated the main effects using a total of 50 naïve subjects. Orientation discrimination around vertical improved after adaptation to either horizontal or vertical gratings, but was impaired by adaptation at 7.5 or 15 degrees from vertical. Improvement was greatest when adapter and test were orthogonal. We show that the results can be understood in terms of a functional model of adaptation in cortical vision.

Possible Neural Substrates for Orientation Analysis and Perception

Recent research into the response properties of extrastriate visual cortical mechanisms has revealed single-cell functional organisation which closely parallels certain global and apparently emergent properties of psychophysical observation. An attempt is made to relate previous data on orientation illusions and aftereffects to these extrastriate mechanisms and new data which cannot be explained adequately by V1 (striate) orientation channels are discussed. Conversely, properties of cells in areas such as V3, V4, MT, and others seem to provide an obvious neural substrate for global interactions. It is suggested that psychological ‘explanations’ couched in terms of ‘hypotheses’ or ‘cognitive problem solving’ lack heuristic value, and that, in contrast, the properties of extrastriate cells can suggest novel experimental psychophysical paradigms which are designed to probe these higher-order global mechanisms more or less selectively.

The depth and selectivity of suppression in binocular rivalry

Binocular rivalry occurs when the two eyes are presented with incompatible stimuli and the perceived image alternates between the two stimuli. The aim of this study was to find out whether the periodic perceptual loss of a monocular stimulus during binocular rivalry is mirrored by a comparable loss of contrast sensitivity. We presented brief test stimuli to one eye while its conditioning stimulus was dominant or suppressed. The test stimuli were varied widely across four stimulus domains—namely, the relative stimulation of medium- and long-wavelength-sensitive cones, duration, spatial frequency, and grating orientation. The result in each case was the same. Suppression depended slightly or not at all on the type of test stimulus, and contrast sensitivity during suppression was around 64% of that during dominance. The effect of suppression on sensitivity is therefore very weak, relative to its effect on the perceived image. Furthermore, suppression was largely independent of the similarity between the conditioning and the test stimuli, indicating that our results are better explained byeye suppression than bystimulus suppression. A model is presented to account for the small, monocular sensitivity loss during suppression: It assumes that test detection precedes conditioning stimulus perception in the visual pathway.

Adaptation to temporal modulation can enhance differential speed sensitivity.

During adaptation to a moving pattern, perceived speed decreases. Thus we know that the adapted visual system does not simply code the absolute speed of a stimulus. We hypothesised that adaptation to a moving stimulus serves to optimise coding of changes in speed at the expense of maintaining an accurate representation of absolute speed. In this case we would expect discrimination of speeds around the adapted level to be preserved or enhanced by motion adaptation. Speed discrimination thresholds were measured for sinusoidal gratings (1.25 cpd; 12.5 Hz; 40% contrast) with and without prior adaptation to moving, static, and flickering stimuli. After adaptation to motion in the same direction as the test, seven of eight subjects showed a reduction of perceived speed in the adapted region, and seven showed enhanced discrimination. Similar effects were found for adaptation to motion in the opposite direction to the test and to counter-phase flicker, suggesting that adaptation is driven by temporal modulation rather than by motion per se. We conclude that motion adaptation preserves or enhances differential speed sensitivity at the expense of an accurate representation of absolute speed.