Orientation Discrimination in a Visual Form Agnosic: Evidence from the McCollough Effect

Configural Gestalts remain nothing more than the sum of their parts in visual agnosia

We report converging evidence that higher stages of the visual system are critically required for the whole to become more than the sum of its parts by studying patient DF with visual agnosia using a configural superiority paradigm. We demonstrate a clear dissociation between this patient and normal controls such that she could more easily report information about parts, demonstrating a striking reversal of the normal configural superiority effect. Furthermore, by comparing DF's performance to earlier neuroimaging and novel modeling work, we found a compelling consistency between her performance and representations in the early visual areas, which are spared in this patient. The reversed pattern of performance in this patient highlights that in some cases visual Gestalts do not emerge early on without processing in higher visual areas. More broadly, this study demonstrates how neuropsychological patients can be used to unmask representations maintained at early stages of processing.

Preserved Striate Cortex is Not Sufficient to Support the McCollough Effect: Evidence from two Patients with Cerebral Achromatopsia

The McCollough effect (ME) is a colour aftereffect contingent on pattern orientation. This effect is generally thought to be mediated by primary visual cortex (V1) although this has remained the subject of some debate. To determine whether V1 is in fact sufficient to subserve the ME, we compared McCollough adaptation in controls to adaptation in two patients with damage to ventrotemporal cortex, resulting in achromatopsia, but who have spared V1. Each of these patients has some residual colour abilities of which he is unaware. Participants performed a 2AFC orientation-discrimination task for pairs of oblique and vertical/horizontal gratings both before and after adaptation to red/green oblique induction gratings. Successful ME induction would manifest itself as an improvement in oblique-orientation discrimination owing to the additional colour cue after adaptation. Indeed, in controls oblique grating discrimination improved post-adaptation. Further, a subdivision of our control group demonstrated successful ME induction despite a lack of conscious awareness of the added colour cue, indicating that conscious colour awareness is not required for ME induction. The patients, however, did not show improvement in oblique-orientation discrimination, indicating a lack of ME induction. This suggests that V1 must be connected to higher cortical colour areas to drive ME induction.

Shape Processing Area LO and Illusory Contours

Recent functional MRI has demonstrated that illusory contours can activate the primary visual cortex. Our investigation sought to demonstrate whether this correlation reflects computations performed in the primary visual cortex or feedback effects from shape processing area LO. We explored this in a patient who has a bilateral lesion to LO, but a functionally spared V1. Our data indicate that illusory contours are unable to influence behaviour without visual area LO. Whilst we would not claim that our data provide evidence for the ‘cognitive’ nature of illusory contours, they certainly suggest that illusory contours are dependent upon the computations involved in extracting shape representations in LO. Our data highlight the importance of neuropsychological research in interpreting the role of feedforward and feedback effects in the generation of visual illusions.

A conscious route to unconscious vision

Damage to the primary visual cortex can leave subjects with unconscious residual vision, or 'blindsight'. New research suggests that 'top-down' modulation by intact conscious visual processes can improve performance in the impaired visual domain, even though that domain still remains quite inaccessible to consciousness.

Motion after-effects and word contingency

Stimulus-selectivity in phenomena such as the McCollough effect and other contingent after effects are controversial. Word specific McCollough effects have been reported (Allan et al., Percept Psychophys 1989;45:104-113) that suggest an associative model rather then a neural one. However, failures to replicate make this finding controversial (Humphrey et al., J Exp Psychol: Gen 123:86-90). We applied the same contingency to the motion after-effect. Moving words, words paired with sine wave gratings and words composed of sine wave gratings failed to generate text contingent after-effects in stimulus situations that normally evoke motion after-effects. Thus, there was little evidence that motion adaptation can be made textually contingent.

The objects of action and perception

Two major functions of the visual system are discussed and contrasted. One function of vision is the creation of an internal model or percept of the external world. Most research in object perception has concentrated on this aspect of vision. Vision also guides the control of object-directed action. In the latter case, vision directs our actions with respect to the world by transforming visual inputs into appropriate motor outputs. We argue that separate, but interactive, visual systems have evolved for the perception of objects on the one hand and the control of actions directed at those objects on the other. This 'duplex' approach to high-level vision suggests that Marrian or 'reconstructive' approaches and Gibsonian or 'purposive-animate-behaviorist' approaches need not be seen as mutually exclusive, but rather as complementary in their emphases on different aspects of visual function.

False categories in cognition: the Not-the-Liver fallacy

This paper reports on an increasingly frequent error committed in cognition research that at best slows progress, and at worse leads to self-perpetuating false claims and misguided research. The error involves how we identify meaningful processes and categories on the basis of data. Examples are given from three areas of cognition: (1) memory, where the misconception has fueled the popular implicit/explicit categories, (2) perception, where the misconception is used to re-evaluate the classic what/where division, and (3) motor skills, where it is used to draw conclusions from patients with Huntington's disease. Reasons for the prevalence of this error, how it relates to double dissociations, and what it suggests about scientific reasoning are offered.

A neurological dissociation between shape from shading and shape from edges

We studied the ability of a neurological patient, who has deficits in various aspects of form perception, to perform region segregation tasks requiring discriminations based on several image properties that are related to the three-dimensional structure of objects. The patient could discriminate the apparent three-dimensional structure and orientation of shapes defined by shading gradients, but could not make such discriminations for shapes in which edges were depicted as lines or as luminance discontinuities. These results suggest that the neural pathways that compute shape from shading gradients may be independent of those that compute shape based on edges, and, based on the patient's pattern of brain damage, they also indicate a relatively early functional separation in the requisite inputs.

Towards the neuronal correlate of visual awareness

Several encouraging developments towards identifying the neuronal correlate of visual awareness have emerged recently. Increasingly sophisticated behavioral paradigms permit the study of visual awareness in humans as well as in non-human primates. In patients with anatomically restricted lesions in striate and extrastriate cortex, highly informative deficits of visual awareness are observed. Similar deficits can be obtained in normal observers with a novel class of psychophysical displays. Taken together, these results suggest that the contents of visual awareness reflect neuronal activity in certain extrastriate, but not in striate, visual cortical areas.

Visual aftereffect of texture density contigent on color of frame

An aftereffect of perceived texture density contingent on the color of a surrounding region is reported. In a series of experiments, participants were adapted, with fixation, to stimuli in which the relative density of two achromatic texture regions was perfectly correlated with the color presented in a surrounding region. Following adaptation, the perceived relative density of the two regions was contingent on the color of the surrounding region or of the texture elements themselves. For example, if high density on the left was correlated with a blue surround during adaptation (and high density on the right with a yellow surround), then in order for the left and right textures to appear equal in the assessment phase, denser texture was required on the left in the presence of a blue surround (and denser texture on the right in the context of a yellow surround). Contingent aftereffects were found (1) with black-and-white scatter-dot textures, (2) with luminance-balanced textures, and (3) when the texture elements, rather than the surrounds, were colored during assessment. Effect size was decreased when the elements themselves were colored, but also when spatial subportions of the surround were used for the presentation of color. The effect may be mediated by retinal color spreading (Pöppel, 1986) and appears consistent with a local associative account of contingent aftereffects, such as Barlow's (1990) model of modifiable inhibition.

The McCollough effect reveals orientation discrimination in a case of cortical blindness

BACKGROUND

The McCollough effect is a colour after-effect that is contingent on the orientation of the patterns used to induce it. To produce the effect, two differently oriented grating patterns--such as a red-and-black vertical grating and a green-and-black horizontal grating--are viewed alternatively for a few minutes. After this period of adaptation, if the black-and-white test gratings are viewed in the same orientation as the adaptation patterns, the white sections of the vertical grating will appear pale green and the white sections of the horizontal grating will appear pink. The McCollough effect indicates that colour- and orientation-coding mechanisms interact at some point during visual processing; but the question remains as to whether this interaction occurs at an early or later stage in the cortical visual pathways. In an attempt to answer this question, we studied a patient who had suffered extensive damage to extrastriate visual areas of the brain, which had left him able to see colour but little else.

RESULTS

Neuropsychological and perceptual tests demonstrated that the patient, P.B., has a profound impairment in form perception and is even unable to discriminate between 90 degrees differences in the orientation of grating stimuli. He is also unable to use orientation information to control his reaching or grasping. Nevertheless, P.B. can name and discriminate different colours reliably, including those used to induce the McCollough effect. After adaptation with red-and-green gratings, P.B. appropriately reported the orientation-contingent aftereffect colours, even though he continued to be unable to discriminate the orientations of the test patterns.

CONCLUSIONS

These results indicate that at some level in P.B.'s visual system orientation is being coded, but it is at a level that he is unable to use in making orientation judgements or in visuomotor control. Given the massive insult to the extrastriate cortex in P.B., it is likely that the anatomical locus of the mechanisms underlying the McCollough effect is within primary visual cortex or even earlier in the visual pathway.

Orientation and color processing for partially occluded objects

The McCollough orientation-contingent color aftereffect could be equally well elicited by either a full test pattern of black and white stripes or a similar test pattern that was largely occluded by white surfaces, provided the latter stripes were made to appear as through continuing under the white surfaces--by means of stereo depth cues. The color aftereffect appeared concentrated around the edges of the stripes that protruded out from under the white surfaces; surfaces that themselves continued to appear a uniform white as shown by color matches. These results suggest that occluded, perceptually-continued edges can elicit the McCollough effect, which is generally thought to occur quite early in the visual pathway.

The role of surface information in object recognition: studies of a visual form agnosic and normal subjects

Three experiments were conducted to explore the role of colour and other surface properties in object recognition. The effects of manipulating the availability of surface-based information on object naming in a patient with visual form agnosia and in two age-matched control subjects were examined in experiment 1. The objects were presented under seven different viewing conditions ranging from a full view of the actual objects to line drawings of those same objects. The presence of colour and other surface properties aided the recognition of natural objects such as fruits and vegetables in both the patient and the control subjects. Experiment 2 was focused on four of the critical viewing conditions used in experiment 1 but with a large sample of normal subjects. As in experiment 1, it was found that surface properties, particularly colour, aided the naming of natural objects. The presence of colour did not facilitate the naming of manufactured objects. Experiment 3 was focused on possible ways by which colour could assist in the recognition of natural objects and it was found that object naming was facilitated only if the objects were presented in their usual colour. The results of the experiments show that colour does improve recognition for some types of objects and that the improvement occurs at a high level of visual analysis.

Text-contingent color aftereffects: a reexamination

Five experiments reexamined color aftereffects contingent on the semantic properties of text (Allan, Siegel, Collins, & MacQueen, 1989). The influence of different assessment techniques and the effect of eye movements and overlapping contour information on the induction of color aftereffects by word and nonword letter strings were determined. Experiment 1 showed that no aftereffect was found when a traditional method of assessing color aftereffects was used. Experiments 2 and 4 demonstrated color aftereffects for both words and nonwords, but only when subjects fixated the same locus during induction and testing and only when assessed with the technique described by Allan et al. (1989). If, however, eye movements were made during induction, no color aftereffect was obtained (Experiment 3). Induction to nontext patterns with properties similar to those of text but with fewer overlapping contours resulted in a strong color aftereffect (Experiment 5). These results suggest that the color aftereffect contingent on text is very weak and is not dependent on semantic factors, but that it is a product of induction to local color and orientation information.

Rapid discrimination of McCollough effects

The McCollough effect is a colour aftereffect that is contingent on pattern orientation. Three experiments were conducted to establish whether such aftereffect colours could serve as a basis for discrimination in several rapid discrimination tasks. In the first experiment it was investigated whether aftereffect colours could act like a simple 'feature' in a visual search task involving a difficult orientation discrimination. Without McCollough adaptation, the time taken to detect a 'target' among 'distractors' increased substantially as the number of distractors increased. With adaptation, detection time was essentially independent of the number of distractors, indicating that the nature of the task changed from a difficult orientation discrimination to a simple discrimination based on differences in aftereffect colours. The second and third experiments employed a difficult four-alternative forced-choice procedure in which subjects were required to discriminate a monochromatic patch of square-wave grating oriented at 45 degrees from three others oriented at 135 degrees (and vice versa). The gratings were presented very briefly (67-333 ms) followed by a 500 ms mask. Subjects performed the task with and without McCollough adaptation. Performance was strikingly better after adaptation: colour aftereffects could be used to make the discrimination even at exposure durations as short as 67 ms. The third experiment demonstrated that this enhanced performance was indeed due to perceived colour differences (rather than a possible contrast difference). The results of the three experiments are discussed in relation to proposals about the locus of the McCollough effect.