What do cases of blindsight tell us about the visual system? Flashcards
Description of Blindsight
In humans, the primary visual cortex (V1) is essential for conscious vision. However, even without V1 and in the absence of awareness, some preserved ability to accurately respond to visual inputs has been demonstrated, a phenomenon referred to as blindsight (Weiskrantz et al, 1974).
Outline of what it can tell us
By studying the visual pathways that underlie this residual vision in patients, we can uncover additional aspects of the human visual system that likely contribute to normal visual function but cannot be revealed under normal physiological conditions.
In this essay, the residual abilities and neural activity that have been described in blindsight and the implications of these findings for understanding the intact system will be discussed.
For the purposes of this essay, it is worth clarifying that the dominant visual pathway projects from…
the retina to V1 via the LGN. However, there are a number of alternative pathways to extrastriate cortex via both the LGN and pulvinar that avoid V1 (Ajina and Bridge, 2016).
Motion abilities
Discriminations between moving stimuli have been shown to be the least affected responses, with discrimination thresholds at or very near those of normal observers (Barbur et al., 1980; Blythe et al., 1986; Perenin, 1991).
Finlay et al., (1997) Patient GY can
detect and localize two moving stimuli presented
simultaneously, one in each hemifield. Sensitivity to motion increases with increasing velocity.
Motion fMRI
Barbur et al (1993)
first to demonstrate that the human motion area (hMT) was activated when patient GY was presented with a moving stimulus to his blindfield.
Further supported by (Bridge et al, 2010; Morland et al, 2004).
These findings suggest that there must be non-striate visual input to hMT that supports neural responses to salient visual motion.
Therefore, V1 is not necessary for all neural responses to visual motion
What underlies the residual motion abilities?
Ajina et al (2015) investigated visual white matter tracts in 17 patients with chronic unilateral V1 damage sustained in adulthood and healthy age-matched controls (n = 9).
The large subject group enabled the division of patients into those demonstrating blindsight, and those who did not.
Three pathways were selected (1) ipsilateral connections between the LGN and hMT+, (2) ipsilateral tracts between the superior colliculus and hMT+, and (3) interhemispheric tracts between hMT+ bilaterally.
The preservation or destruction of the geniculate-hMT+ tract predicted presence or absence of blindsight respectively.
More specifically, the geniculate-hMT+ tract was reliably identified in all blindsight positive patients, but was impossible to track or showed considerably impaired white matter microstructure in all blindsight negative individuals.
In contrast, the two alternative candidate tracts showed variable predominance in both patient groups and therefore seem unlikely to underlie blindsight function.
This suggests that an ipsilesional connection between LGN and hMT is critical for residual motion abilities in blindsight
Tells us that there is a direct pathway between LGN and hMT, not just LGN→ V1→ V5
Parallel processing in the normal system
Shape and form in blindsight
(Marcel, 1998). There are examples of successful shape discrimination in patients with blindsight
(Morris et al, 2001). Could perform gender identification for faces in his blind field
Trevethan et al (2007) found that patient DB could successfully identify line low-contrast images of objects, make successful same/different discriminations for pairs of stimuli presented in his blindfield and identify complex images presented entirely in his blindfield.
This suggests that ventral regions of extrastriate cortex can receive visual input in the absence of V1- perhaps via direct diffuse pulvinar projections or via interhemispheric callosal connections (Van Essen et al, 1982).
TMS to V1 and orientation/colour discrimination
Boyer et al (2005) used TMS to deactivate V1
Despite unawareness of visual targets, performance on forced choice discrimination tasks for orientation and colour were significantly above chance
Suggests that a functioning geniculo extrastriate visual pathway that bypasses V1 exists which can process orientation and colour in the absence of conscious awareness
Guidance of the motor system can be driven in the absence of V1…
Gripping objects
(Danckert & Rossetti, 2005).
The ability of hemianopic patients to locate targets either by saccades or pointing was one of the first indications of blindsight and this type of motor response has been coined ‘action blindsight’
Whitwell et al (2011) determined that while a hemianopic patient was able to accurately grip objects presented in the blind field, the patient could not perform this task when a delay was inserted after stimulus presentation.
This suggests that online perception for action has a different mechanism than memory for object size, which appears to require V1, a finding supported in fMRI in healthy PS (Singhal et al, 2013).
an extreme example of action blindsight…
De Gelder et al (2008)
demonstrated that TN was able to navigate along a corridor avoiding obstacles, despite being unable to see
Suggests that the motor system has access to visual information that is not consciously perceived which doesn’t pass through V1
A pathway underlying such information flow likely includes the posterior parietal cortices which are involved in the visual guidance of movement (Culham et al, 2006)
Patient GY is able to successfully discriminate different facial expressions in his blind field from videos of a female face and shows increased bilateral amygdala activity for fearful faces in his blind hemifield
de Gelder et al, 1999
Morris et al, 2001
GY also showed _______ for angry vs neutral ______ actions
Van den Stock et al, 2011
bilateral superior colliculus, pulvinar, amygdala and right fusiform gyrus activity for angry verses neutral whole-body actions
Patient TN can discrimination _____ or ____ facial expressions inside his scotoma and his ______ shows activity in response to faces depicting ____, ____ or _____ compared to ____ expressions
Pegna et al, 2005
Patient TN can discriminate positive or negative facial expressions inside his scotoma and his right amygdala shows activity in response to faces depicting anger, happiness or fear compared to neutral expressions.
what does affective blindsight teach us?
These results supports a role for the amygdala in unconscious processing of emotion and implicates additions structures such as the superior colliculus and pulvinar
This raises the possibility of a specific subcortical pathway that might underlie affective blindsight
Suggests that affective visual information doesn’t need to pass through V1 which normally process affective information
Spectral sensitivity
Stoerig & Cowey (1989, 1991, 1992) show that spectral sensitivity function can be qualitatively normal in the blind field, although with quantitatively reduced sensitivity.
- Spectral sensitivity profile includes humps and troughs reflecting colour opponency
- Also shows the characteristic loss of long wavelength sensitivity following dark adaptation
- Purkinje shift
- Wavelength discrimination is also possible using FC guessing
