Thursday, 13 October 2011

Week Three - Visual Perception

Visual perception is a person’s eye sight it allows the sensory and perceptual systems to interpret information from their environment and therefore begin the process of how to react to the visual stimuli before them. Brain damage and lesions to the visual system consequently result in a severe deficit to a person’s visual abilities and can cause many perceptual disorders. These include; Agnosias, visual agnosias and prosopagnosia. Agnosia is an impaired ability to recognise basic objects, this is however not attributed to sensory or attentional deficits. The most common cause of agnosia is due to brain damage causing sensations such as light and colour to become confused. Interestingly patients with agnosia that cannot recognise an object can more than likely specify what the object is if they are allowed to touch it, therefore showing that the patient has an intact memory about the object, but just cannot visually recognise it. Visual agnosia is caused by right inferior parietal lobe lesions. Patients with visual object agnosia may experience symptoms such as difficulty with copying drawings, and finding it hard to match objects . Stroke and heart attack can cause this perceptual disorder. Prosopagnosia patients find it difficult to recognise faces, however the ability to recognise objects stays intact. Interestingly it was reported by Hecaen & Angelergues (1962) that patients with this disorder have difficulty recognising the faces of people they previously knew or even pictures of their own face. Although prosopagnosia patients have an inability to recognise faces they can however still identify individuals by peripheral cues such as voice, hair, silhouette etc. It has been recognised that the reason for some patients to report not being able to recognise faces but still being able to recognise objects or vice versa is due to object and face recognition activating different parts of the brain.

Questions for week three;

  • Q1 - Why do we need to move our eyes across a scene?
- The movement of our eyes helps direct focus and attention to stimuli of high salient importance, therefore making it easier to take in visual information and process it in the brain.

  • Q2 - Would it not be easier if we could see the whole scene in front of us at once?
- This would not be ideal because the brain cannot cope with an overload of visual information, therefore it is best to focus attention on information of high salience.

  • Q3 - What does FEF mean? And what is its role in vision?
- FEF means frontal eye field and it is an area in the frontal cortex involved in the generation of motor commands for pointing the eyes , and therefore the foveas toward desired target locations.

Week 3 article reading - 'Cognitive Physiology: Moving the mind's eye before the head's eye'.

- It is said that when we shift our spatial attention it is most often followed by a matching eye movement. Evidence has suggested that this reflects the ability of the same cortical area to shift eye position and the locus of attention.
-It has been suggested that the visual system has evolved greatly over time and in a very selective way. It has been noted that the most evolved is the high spatial resolution being confined to a small fraction of the retina, the fovea. This is very instrumental as if the abilities of the fovea were spread across the whole retina this would have been difficult, and resulted in a flood of information reaching the brain, which would have been impossible to accommodate.
-The visual system now identifies the object and reflexively or voluntarily foveates them for detailed analysis, therefore a dedicated motor system has been developed. The motor system is only activated when a worthwhile target for an eye movement has been identified. This all highlights how much the human brain has evolved.

-Because of an overload of information arriving at the eyes with little of this being of importance, an attentional system has been developed.

-Voluntary attention enhances the activity of neurons in extrastriate cortex when something seems of importance.

-The central mechanism that directs attention still seems unclear because although the link between extrafoveal attentional location & eye movement appears strong it has not been shown that one causes the other.

-However Moore& Armstrong tried to demonstrate that there was a link. In a study with monkeys they found that the absolute response enhancement was stronger if presented with a preferred stimulus then when presented with a non-preferred stimulus, showing that attention is directed at things that are appealing to us.

-Findings have found that the FEF plays a central role in directing spatial attention and that this process is linked directly to the generation of eye movement commands.

-Stimulation of the FEF shifts attention.

-A criticism of this theory is that attention can also be directed to a low-salience portion of the visual field which can cause eye movements.

-Attention can also be directed at features as well as locations.

-It seems that the FEF plays the main role in stimulus salience, however other cortical areas are also involved in carrying salient signals.

Important info from lecture 3

Two cortical routes = what & where
The two crucial functions are;

Object Perception = Where is it?
  • A lesion to this part of the brain causes optic ataxia, resulting in problems with reaching and grasping things.
Saccades = Rapid eye movements (lasts between 25 & 100 msec)
Fixations = Pauses between saccades where the eye takes in visual information.

Recognition & response are filters of attention - Hearing, seeing & touching can aid attention.

Attention impairments

Agnosia = Unattended stimuli is not available to conscious awareness.

Agnosias: Impairments of recognition
It's the inability to recognize basic objects, this is caused by brain damage.

Object agnosia

Patients cannot recognise common objects, yet have no problem in recognizing faces.

  • Some patients have trouble recognizing common objects whilst others have trouble recognizing faces, suggesting that objects & faces may be processed by separate perceptual mechanisms/brain areas.
Prosopagnosia ('face blindness')
  • Although patients have basic recognition they cannot recognise faces.
  • With object agnosia- visual acuity is spared.
  • Because patients with prosopagnosia have difficulty recognising faces they often rely on other factors such as hair, voice or other peripheral cues.
  • In an experiment using an FMRI machine, it showed that when patients viewed pictures of objects their PPA became more active and when viewing faces their FFA became more active, showing that faces and objects use different parts of the brain.

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