What do lesions of V4 tell us about the role of v4? Flashcards
Intro
The ventral surface of the human occipital lobe contains multiple retinotopic maps.
The most posterior of these maps, the ‘4th’ map, is termed V4.
It was first discovered in the macaque and was referred to as a “colour area” (Zeki, 1973)
In humans, an area in the vicinity of the lingual and fusiform gyri responds selectively to chromatic stimuli (Zeki et al, 1991)
Damage to this area results in ‘cerebral achromatopsia’ - an inability to perceive colour (Heywood et al, 1991).
The exact nature of lesioning v4 continues to be the subject of considerable investigation and debate over the last 25 years.
This essay will explore the effects of lesioning V4, in terms of impairments and residual abilities and will discuss the implications of these findings in relation the function of V4.
It is worth noting that the exact location of V4 in the human brain remains a subject of intense debate. However, for the purposes of this essay, it is assumed that hV4 refers to the 4th visual map located on the ventral surface of the occipital lobe.
Lesioning V4 results in very qualitatively different impairments compared to lesioning V1-V3…
When V1-V3 is lesioned, patients have partial visual field blindness (scotomas) (Inouye, 1909)
However, lesions to V4 do not and instead cause selective perceptual deficits (Bouvier and Engel, 2006)
Therefore, lesioning V4 does not lead to visual field blindness but instead higher-order processing impairments.
This suggests that V4 likely plays an important role in coordinating signal between these early retinotopic maps and the various downstream visual areas involved in recognition and appearance.
This is further supported by the posterior location of V4
Lesioning V4 results in impairments in colour discrimination and identification
Heywood et al (1991,1994) Investigated patient MS who has bilateral occipitotemporal damage which includes V4
Performs randomly on tasks of color ordering and is unable to select an oddly colored patch embedded in an array of differently colored equiluminant hues.
Yet, importantly, he has no difficulty in locating each of the targets and distractors presented against an equiluminant background.
Edges defined by pure color differences remain visible to him.
Therefore, lesioning V4 results in impairments with colour identification and discrimination but the ability to detect colour stimuli against achromatic stimuli remains
This suggests that V1 allows for very basic colour processing abilities such as colour detection but that v4 is necessary for higher order abilities like identification/discrimination.
Lesioning V4 in humans results in difficulty with colour constancy tasks
Kennard et al (1995) Patient BL
The perceived colour of surfaces changes with changes in the wavelength composition and deviates towards the colour of the dominant wavelength reflected for the surface.
In normal subjects, this does not occur and this ability is referred to as colour constancy
It appears, therefore, that V4 is critical for the ratio-taking operations that enable the brain to compare the wavelength composition of the light
Therefore, V4 is likely to be the top of the colour processing hierarchy and facilitates colour constancy
However, this explanation assumes that the colour system is hierarchically organised which is discussed further below.
Patients with lesions to V4 are able to use colour information to detect motion
Cavanagh et al (1998) 3 PS with bilateral damage to V4
PS could detect motion at high colour contrasts equivalent to normal subjects
At high colour contrasts, the strength of the colour contribution to motion was equivalent to normal subjects
Therefore, it appears that there is a direct pathway for colour information from the retina to cortical motion detectors that is used in patients with v4 lesions
Therefore, colour information does not flow as a single hierarchically organised stream through the visual system and not all the information passes through V4
Instead, colour information appears to project in a parallel manor to different processing centres and it appears only one of these centres (v4) leads to the conscious perception of colour
‘patchwork’ model
(Haan and Cowey, 2011) argues that depending on the goal certain visual areas are recruited that process information relevant for the task at hand
I.e. in this case, V4 is not needed for the perception of colour information necessary for motion detection
Therefore, the term ‘colour centre’ for V4 may be misleading as it implies all colour related processing must and does pass through V4
A hierarchical organisation of colour processing which has V4 at the top, requires a much more elaborate Darwinistic approach compared to the view that different visual abilities evolved independently.
For there to be a single true colour centre, the damaged region in achromatopsia patients should show three properties…
It should contain a single visual area
Colour vision should be the only perceptual ability it supports
Colour vision should not be critically dependent upon other late visual areas
Evidence against these three assumptions….
Bouvier and Engel (2006)
They investigated the behavioural and neural characteristics of 92 cases of achromatopsia and 100 cases of prosopagnosia
Behavioural results from the achromatopsia patients revealed that damage to a small localised area results in impaired colour naming and recognition.
This provides strong evidence that this region is important for colour vision
Firstly, the authors pointed out that the region of common overlap in this study likely contained two retinotopically defined visual areas.
Secondly, the common vision also appeared important for spatial vision, since spatial deficits almost always co-occur with achromatopsia.
Third, other late visual areas may play a role in colour perception as there was frequently substantial residual colour vision even when the common region was damaged.
Therefore, their results support a less centralised view in which colour perception arises from a stream of processing that flows through multiple multipurpose visual areas.
As such, V4 cannot be termed the ‘colour centre’…
Conclusion
lesions to V4 result in impairment in the perception and processing of colour. Firstly, it leads to issues with the conscious perception of colour (e.g. discrimination of colours). Secondly, it leads to issues in the processing of colour (e.g. the computations needed for colour constancy). Taken together, these findings suggest that the processing colour system is closely related to the perceptual system (Zeki and Bartels, 1998). While the idea of V4 being the ‘colour centre’ of the brain at the top of a hierarchical pathway is an alluring one, the residual colour abilities of achromatopsia patients suggests that other visual regions and pathways also support colour processing. Indeed, a patchwork account of colour processing seems more likely. Thus, V4 is a critical hub of colour processing but not the only one.
