Tag Archives: ventral stream

Some visual-form areas are really task areas

There are two paths for visual information, one to the motor areas (dorsal ‘where’ stream) and one to the areas concerned with consciousness, memory and cognition (ventral ‘what’ stream). The visual ventral stream has areas for the recognition of various categories of object: faces, body parts, letters for example. But are these areas really ‘visual’ areas or can they deal with input from other senses? There is recent research into an area concerned with numerals. (see citation below) There are some reasons to doubt a ‘vision only’ processing in these areas. “…cortical preference in the ‘visual’ cortex might not be exclusively visual and in fact might develop independently of visual experience. Specifically, An area showing preference for reading, at the precise location of the VWFA (visual word-form area), was shown to be active in congenitally blind subjects during Braille reading large-scale segregation of the ventral stream into animate and inanimate semantic categories have also been shown to be independent of visual experience. More generally, an overlap in the neural correlates of equivalent tasks has been repeatedly shown between the blind and sighted using different sensory modalities.” Is an area specialized in one domain because of cultural learning through visual experience or is the specialization the result of the specific connectivity of an area?

Abboud and others used congenitally blind subjects to see if the numeral area could process numerals arriving from auditory signals. Congenitally blind subjects cannot have categorical area that are based on visual learning. The letter area and numeral area are separate even though the letter symbols and numeral symbols are very similar – in fact can be identical. The researchers predicted that the word area had connections to language areas and the numeral area connected to quantitative areas.

eye-music application

eye-music application

The subjects were trained in eye-music, a sight substitute based on time, pitch, timbre and volume. While being scanned, the subjects heard the same musical description of an object and were asked to identify the object as part of a word, part of a number, or a colour. Roman numerals were used to give a large number of identical musical descriptions of numbers and letters. What they found was that the numeric task gave activation in the same area as it does in a sighted person and that blind and sighted subjects had the same connections, word area to language network and numeral area to quantity network. It is the connectivity patterns, independent of visual experience, that create the visual numeral-form area. “…neither the sensory-input modality and visual experience, nor the physical sensory stimulation itself, play a critical role in the specialization observed in this area. ” It is which network is active (language or quantity) that is critical.

…these results are in agreement with the theory of cultural recycling, which suggests that the acquisition of novel cultural inventions is only feasible inasmuch as it capitalizes on prior anatomical and connectional constraints and invades pre- existing brain networks capable of performing a function sufficiently similar to what is needed by the novel invention. In addition, other factors such as the specifics of how literacy and numeracy are learned, as well as the distinctive functions of numerals and letters in our education and culture, could also account for the segregation of their preferences.

Here is the abstract: “Distinct preference for visual number symbols was recently discovered in the human right inferior temporal gyrus (rITG). It remains unclear how this preference emerges, what is the contribution of shape biases to its formation and whether visual processing underlies it. Here we use congenital blindness as a model for brain development without visual experience. During fMRI, we present blind subjects with shapes encoded using a novel visual-to-music sensory-substitution device (The EyeMusic). Greater activation is observed in the rITG when subjects process symbols as numbers compared with control tasks on the same symbols. Using resting-state fMRI in the blind and sighted, we further show that the areas with preference for numerals and letters exhibit distinct patterns of functional connectivity with quantity and language-processing areas, respectively. Our findings suggest that specificity in the ventral ‘visual’ stream can emerge independently of sensory modality and visual experience, under the influence of distinct connectivity patterns. ”
ResearchBlogging.org

Abboud, S., Maidenbaum, S., Dehaene, S., & Amedi, A. (2015). A number-form area in the blind Nature Communications, 6 DOI: 10.1038/ncomms7026

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Unconscious vision

Milner (see citation below) reviews the evidence that the visual-motor control is not conscious.

 

Visual perception starts at the back of the optical lobe and moves forward in the cortex as processing proceeds. There are two tracks along which visual perception proceeds, called the dorsal stream and the ventral stream. The two streams have few interconnections. The dorsal stream runs from the primary visual cortex to the superior occipito-parietal cortex near the top the the head. The ventral stream runs from the primary visual cortex to the inferior occipito-temporal cortex at the side of the head. Their functions, as far as is known, differ. “The dorsal stream’s principal role is to provide real-time ‘bottom-up’ visual guidance of our movements online. In contrast, the ventral stream, in conjunction with top-down information from visual and semantic memory, provides perceptual representations that can serve recognition, visual thought, planning and memory offline… we have proposed that the visual products of dorsal stream1 processing are not available to conscious awareness—that they exist only as evanescent raw materials to provide the unconscious moment-to-moment sensory calibration of our movements.

 

The researchers used three methods in their studies: patients with lesions in their visual system, patients suffering from visual extinction, and fMRI experiments.

 

One patient had part of their ventral streams destroyed – they could reach and grasp objects that they were not conscious of. The opposite was true of other patients with damage to their dorsal streams – they had difficulties grasping objects that they were consciously aware of.

 

Visual extinction is a form of spatial neglect. The patient fails to detect a stimulus presented on the side of space opposite the brain damage when and only when there is simultaneously a stimulus on the good side. By carefully arranging an experimental setup, a patient with visual extinction took account of an obstacle that they were not conscious of when reaching for an object. Avoiding an obstacle depends of the dorsal stream because patients with damage to the dorsal stream did not adjust their reaching movements in the presence of obstacles.

 

There is visual feedback during reaching. “Under normal viewing conditions, the brain continuously registers the visual locations of both the reaching hand and the target, incorporating these two visual elements within a single ‘loop’ that operates like a servomechanism to progressively reduce their mutual separation in space (the ‘error signal’) as the movement unfolds. When the need to use such visual feedback is increased by the occasional introduction of unnoticed perturbations in the location of the target during the course of a reach, a healthy subject will make the necessary adjustments to the parameters of his or her movement quite seamlessly. ..In contrast, a patient with damage to the dorsal

 

stream was quite unable to take such target changes on board: she first had to complete the reach towards the original location, before then making a post hoc switch to the new target location…It thus

 

seems very likely that the ability to exploit the error signal between hand and target during reaching is dependent on the integrity of the dorsal stream.

 

The phenomenon of binocular rivalry where the subject has different images projected to the two retinas and is alternatively conscious of one or the other image has been studied with fMRI. It is possible to see which image is conscious by the activity in the ventral stream. But the dorsal stream is able to act on information even if it is not being processed by the ventral stream and therefore not consciously available.

 

The authors do point out that they are not saying that the dorsal stream plays no role in conscious perception. It may for example have some control over attention.

 

In the conclusion, they say “according to the model, such ventral-stream processing plays no causal role in the real-time visual guidance of the action, despite our strong intuitive inclination to believe otherwise (what Clark calls ‘the assumption of experienced-based control’). According to the Milner & Goodale model, that real-time guidance is provided through continuous visual monitoring by the dorsal stream of those very same visual inputs that we experience by courtesy of our ventral stream.

ResearchBlogging.org

A.D. Milner (2012). Is visual processing in the dorsal stream accessible to consciousness? Proc R Soc B, 2289-2298 DOI: 10.1098/rspb.2011.2663

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