Visual and two bottom magnocellular layers which
Visual perception can be easily described as how we perceive reality and objects through our vision and how we process this information in our brain. Vision is our primary sense meaning it is the sense which is used most frequently and is processed from the external world to our retina to our primary visual cortex which has two visual streams.
The journey through the external world to our cortex is taken in by many features of the eye including the Iris which regulates the amount of light reaching the retina and the Pupil which is the hole in the centre of your Iris that allows the light to enter the retina. From the retina to the primary cortex there are Magno and Parvo channels which consist of four top parvocellular layers which are small cell bodies and two bottom magnocellular layers which are large cell bodies. The top four layers detect stationary and slow moving objects and the bottom 2 layers detect movement. This is further processed by the Brain and it’s many lobes consisting of Occipital, Parietal and Temporal which are all used in vision. The Occipital lobe, however, is the main lobe associated with vision as it is known as the visual processing center and consists of the primary visual cortex or V1 for short.
As found by Hubel and Wiesel, V1 has a columnar organization consisting of receptive fields which sees it possible for a visual stimulus to influence a firing on that neuron. The visual cortex consisting of the primary visual cortex, the secondary cortex and visual association cortex then splits to the two visual streams, the dorsal stream which uses vision for action and the ventral stream which uses vision for perception. An example of a brain function that is linked to visual perception is the amygdala which is located deep in the brain as two sets of neurons which allocates value to an object through vision, for example, a tiger would emit a fear response therefore you would be told by your brain to run away from the tiger. This example is explained by Lavengin (2012) as they say ‘For instance, the sight of palatable food raises the blood pressure of primates, a response that disappears after destructive lesions to the amygdala’ and goes on further to say ‘In essence, the amygdala helps assigning positive or negative emotions to a given context’ showing how the amygdala is both a pleasure and fear receptor. This evidence is further supported by the explanation of the amygdala by Edwards (2005) ‘The amygdala, from the Greek word for almond, controls autonomic responses associated with fear, arousal, and emotional stimulation and has been linked to neuropsychiatric disorders, such as anxiety disorder and social phobias’. However to understand deeper how brain functions are linked to visual perception, different studies throughout many years must be discussed and how works of Mishkin.
Ungerleider and Macko (1982) have been expanded upon by Goodale and Milner (1992) and how many other studies both in earlier and more recent times link visual perception to brain functions. Earlier studies on visual perception saw there being only one visual system until Schneider (1967) looked at the fact of there being two visual systems as he orchestrated work by inspecting lesions (tissue which has been damaged) on the visual cortex of the adult Syrian Golden Hamster as they have properties similar to the human brain. Schneider used methods of heat to damage the skull in which the midbrain was totally or partially damaged in 16 of the new born hamster pups.
After 12 weeks of a normal life of a hamster a visual test was performed using sunflower seeds being held and the results showed that the midbrain that was damaged still functioned normally and the hamsters were able to see the seeds with little hesitation due to a recovered midbrain being able to fix the orientation deficit given at birth. However, if the lesion were given at a later age in life, the hamsters midbrain could not fully recover and when the sunflower seed was held above them the hamster would just stare and not be able to view the seed due to the lesion being given to the midbrain at a later age and the development of the visual cortex not being able to repair itself. This led Schneider to discover there was more than one visual system as the hamsters could not perceive their orientation but could still have vision and if lesions were given at birth the midbrain and orientation of the hamsters would recover. The amygdala isn’t taken into too much consideration in this study, however it paths the way forward for other studies to develop upon the role and functions of the amygdala.Schneider’s study on visual perception led to further development by Mishkin, Ungerleider and Macko in 1982 who looked at different types of visual processing in different parts of the brain. To help expand upon previous studies, Mishkin et al used monkeys to experiment on instead of hamsters as they were primates and closer to humans both physically and mentally.
These experiments consisted of implanting lesions in both the posterior parietal lobe which was found to form landmark discrimination and the inferior temporal which was found to form object discrimination. This was similarly tested by Mountcastle et al (1975) who tested neurons in monkey’s brains and how different neurons related to different visual performances in the dorsal stream, this however didn’t take into account the size of the objects and how this affected neurons. Mishkin et al looked at the idea of ‘What vs Where’ meaning what is the object and where the object is. Mishkin et al improved upon the work of studies by Mountcastle and Schneider by exploring further the role of the brain functions in visual perception by exploring how different functions depend on where abouts the brain was damaged.
However, Mishkin et al never took into account the coalition of object and spatial information as it was only a theory and not fully tested. Mishkin et al (1982) concluded about visual processing by saying that ‘appreciation of an objects qualities and of it’s spacial location depends on processing of different kinds of visual information in the inferior temporal and posterior parietal cortex respectively’ coming to the fact that these diverse sections of the brain play different roles in visual perception. This can also relate back to the amygdala and how you identify an object using ‘What vs Where’ as an example. This further explains that the anatomy of ventral and dorsal streams output from V1 is connected and each stream receives input from magno and parvo pathways from vision of the external world. Another key study in understanding how visual perception relates to brain functions is Goodale and Milner (1992) who expand upon Mishkin et al (1982) by looking further into visual pathways. They achieved this by testing on different patients testing both the dorsal and ventral streams using Mishkin et al’s object and landmark study as influence to progress their study. Goodale and Milner also expanded upon Mishkin et al’s ‘What vs Where’ by researching further into the human brain by using human subjects and creating their own view of ‘What vs How’ using ideas of what an object is (identical to Mishkin et al) but instead of where the object is, Goodale and Milner looked at how to get to it.
In their study, Goodale and Milner used human subjects with damage to one projection system with a patient who had optic ataxia which is explained by Pisella et al as ‘a disorder that results from either unilateral or bilateral damage to the caudal part of the posterior parietal cortex (PPC)’. As Mishkin et al discovered, the posterior parietal cortex is used for landmark processing which means damage to this area would give difficulty to knowing exactly where an object is. With this patient , the study consisted of picking up a small wooden block which would change in size in each experiment. The results showed there was difficulty for the patient perceiving how to grasp the object as many adjustments were made when attempting to grab the object which was rarely seen in normal subjects. They also studied another patient who had trouble with moving a stylus through a maze however didn’t struggle with moving their whole body through the maze with a two dimensional map. The dorsal stream from V1 to the posterior parietal region mediates our movements at these objects.
Goodale and Milner also tested on a patient known as patient DF who had visual agnosia (being able to see but not fully recognise objects) from carbon monoxide poisoning. A study similar to the one conducted on the patient with optic ataxia with different sized blocks in which accuracy of hand and finger movements was high, however, could not distinguish differences of the dimensions of the blocks. The study showed that DF could pick up the different sized blocks fairly easily but couldn’t perceive the dimensions of the blocks. A similar study with a slot and card showed comparable results to the block test as DF couldn’t perceive the slot’s position, however, could physically move her hand or card into the slot but using orientation which shows how the ventral stream from striate complex to inferotemporal cortex is used to identify objects.