The full extent of the brain's ability to compensate for damage or changed experience is yet to be established. One question particularly important for evaluating and understanding rehabilitation following brain damage is whether recovery involves new and aberrant neural connections or whether any change in function is due to the functional recruitment of existing pathways, or both. Blindsight, a condition in which subjects with complete destruction of part of striate cortex (VI) retain extensive visual capacities within the clinically blind field, is an excellent example of altered visual function. Since the main pathway to the visual cortex is destroyed, the spared or recovered visual ability must arise from either an existing alternative pathway, or the formation of a new pathway. Using diffusion-weighted MRI, we show that both controls and blindsight subject GY, whose left VI is destroyed, show an ipsilateral pathway between LGN (lateral geniculate nucleus) and human motion area MT+/V5 (bypassing VI). However, in addition, GY shows two major features absent in controls: (i) a contralateral pathway from right LGN to left MT+/V5, (ii) a substantial cortico-cortical connection between MT+/V5 bilaterally. Both observations are consistent with previous functional MRI data from GY showing enhanced ipsilateral activation in MT+/V5. There is also evidence for a pathway in GY from left LGN to right MT+/V5, although the lesion makes its quantification difficult. This suggests that employing alternative brain regions for processing of information following cortical damage in childhood may strengthen or establish specific connections.

Changes in connectivity after visual cortical brain damage underlie altered visual function. Commentary

Trust is a key aspect of various social interactions. Correspondingly, trust has been heavily studied across different scientific disciplines. However, an integration of the diverse research and li...

Trust: An integrative review from a person–situation perspective.

Homonymous hemianopias: clinical-anatomic correlations in 904 cases

The interocular suppression technique termed continuous flash suppression (CFS) has become an immensely popular tool for investigating visual processing outside of awareness. The emerging picture from studies using CFS is that extensive processing of a visual stimulus, including its semantic and affective content, occurs despite suppression from awareness of that stimulus by CFS. However, the current implementation of CFS in many studies examining processing outside of awareness has several drawbacks that may be improved upon for future studies using CFS. In this paper, we address some of those shortcomings, particularly ones that affect the assessment of unawareness during CFS, and ones to do with the use of "visible" conditions that are often included as a comparison to a CFS condition. We also discuss potential biases in stimulus processing as a result of spatial attention and feature-selective suppression. We suggest practical guidelines that minimize the effects of those limitations in using CFS to study visual processing outside of awareness.

/pdf/on-the-use-of-continuous-flash-suppression-for-the-study-of-9w8dii8swn.pdf

On the use of continuous flash suppression for the study of visual processing outside of awareness

Although damage to the primary visual cortex (V1) causes hemianopia, many patients retain some residual vision; known as blindsight. We show that blindsight may be facilitated by an intact white-matter pathway between the lateral geniculate nucleus and motion area hMT+. Visual psychophysics, diffusion-weighted magnetic resonance imaging and fibre tractography were applied in 17 patients with V1 damage acquired during adulthood and 9 age-matched controls. Individuals with V1 damage were subdivided into blindsight positive (preserved residual vision) and negative (no residual vision) according to psychophysical performance. All blindsight positive individuals showed intact geniculo-hMT+ pathways, while this pathway was significantly impaired or not measurable in blindsight negative individuals. Two white matter pathways previously implicated in blindsight: (i) superior colliculus to hMT+ and (ii) between hMT+ in each hemisphere were not consistently present in blindsight positive cases. Understanding the visual pathways crucial for residual vision may direct future rehabilitation strategies for hemianopia patients.

/pdf/human-blindsight-is-mediated-by-an-intact-geniculo-2lb7l3fjo4.pdf

Human blindsight is mediated by an intact geniculo-extrastriate pathway

It has been proposed that two major axes, dominance and trustworthiness, characterize the social dimensions of face evaluation. Whether evaluation of faces on these social dimensions is restricted to conscious appraisal or happens at a preconscious level is unknown. Here we provide behavioral evidence that such preconscious evaluations exist and that they are likely to be interpretations arising from interactions between the face stimuli and observer-specific traits. Monocularly viewed faces that varied independently along two social dimensions of trust and dominance were rendered invisible by continuous flash suppression (CFS) when a flashing pattern was presented to the other eye. Participants pressed a button as soon as they saw the face emerge from suppression to indicate whether the previously hidden face was located slightly to the left or right of central fixation. Dominant and untrustworthy faces took significantly longer time to emerge (T2E) compared with neutral faces. A control experiment showed these findings could not reflect delayed motor responses to conscious faces. Finally, we showed that participants' self-reported propensity to trust was strongly predictive of untrust avoidance (i.e., difference in T2E for untrustworthy vs neutral faces) as well as dominance avoidance (i.e., difference in T2E for dominant vs neutral faces). Dominance avoidance was also correlated with submissive behavior. We suggest that such prolongation of suppression for threatening faces may result from a passive fear response, leading to slowed visual perception.

/pdf/unconscious-evaluation-of-faces-on-social-dimensions-22qdb5w0ev.pdf

Unconscious evaluation of faces on social dimensions.

Motion area V5/MT+ shows a variety of characteristic visual responses, often linked to perception, which are heavily influenced by its rich connectivity with the primary visual cortex (V1). This human motion area also receives a number of inputs from other visual regions, including direct subcortical connections and callosal connections with the contralateral hemisphere. Little is currently known about such alternative inputs to V5/MT+ and how they may drive and influence its activity. Using functional magnetic resonance imaging, the response of human V5/MT+ to increasing the proportion of coherent motion was measured in seven patients with unilateral V1 damage acquired during adulthood, and a group of healthy age-matched controls. When V1 was damaged, the typical V5/MT+ response to increasing coherence was lost. Rather, V5/MT+ in patients showed a negative trend with coherence that was similar to coherence-related activity in V1 of healthy control subjects. This shift to a response-pattern more typical of early visual cortex suggests that in the absence of V1, V5/MT+ activity may be shaped by similar direct subcortical input. This is likely to reflect intact residual pathways rather than a change in connectivity, and has important implications for blindsight function. It also confirms predictions that V1 is critically involved in normal V5/MT+ global motion processing, consistent with a convergent model of V1 input to V5/MT+. Historically, most attempts to model cortical visual responses do not consider the contribution of direct subcortical inputs that may bypass striate cortex, such as input to V5/MT+. We have shown that the signal change driven by these non-striate pathways can be measured, and suggest that models of the intact visual system may benefit from considering their contribution.

Motion area V5/MT+ response to global motion in the absence of V1 resembles early visual cortex.

When the primary visual cortex (V1) is damaged, the principal visual pathway is lost, causing a loss of vision in the opposite visual field While conscious vision is impaired, patients can still respond to certain images; this is known as ‘blindsight’ Recently, a direct anatomical connection between the lateral geniculate nucleus (LGN) and human motion area hMT+ has been implicated in blindsight However, a functional connection between these structures has not been demonstrated We quantified functional MRI responses to motion in 14 patients with unilateral V1 damage (with and without blindsight) Patients with blindsight showed significant activity and a preserved sensitivity to speed in motion area hMT+, which was absent in patients without blindsight We then compared functional connectivity between motion area hMT+ and a number of structures implicated in blindsight, including the ventral pulvinar Only patients with blindsight showed an intact functional connection with the LGN but not the other structures, supporting a specific functional role for the LGN in blindsight

Blindsight relies on a functional connection between hMT+ and the lateral geniculate nucleus, not the pulvinar

Damage to the primary visual cortex removes the major input from the eyes to the brain, causing significant visual loss as patients are unable to perceive the side of the world contralateral to the damage. Some patients, however, retain the ability to detect visual information within this blind region; this is known as blindsight. 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 physiological conditions. In this review, we discuss the residual abilities and neural activity that have been described in blindsight and the implications of these findings for understanding the intact system.

/pdf/blindsight-and-unconscious-vision-what-they-teach-us-about-2sixh4ptib.pdf

Sara Ajina

Papers

Human blindsight is mediated by an intact geniculo-extrastriate pathway

Unconscious evaluation of faces on social dimensions.

Motion area V5/MT+ response to global motion in the absence of V1 resembles early visual cortex.

Blindsight relies on a functional connection between hMT+ and the lateral geniculate nucleus, not the pulvinar

Blindsight and Unconscious Vision: What They Teach Us about the Human Visual System: