PHY3111 Mid-Semester Test Study. Lecture 2: The hierarchical organisation of vision

Transcription

1 PHY3111 Mid-Semester Test Study Lecture 2: The hierarchical organisation of vision 1. Explain what a hierarchically organised neural system is, in terms of physiological response properties of its neurones. - Hierarchical processing, this idea that you have to start with single cells in your retina that see just a small spot of the image, and then gradually be putting together images which are progressively bigger and meaningful in a way that allows you to interpret it as an object. - For e.g. this hierarchical processing of visual processing involves steps such as:: Photoreception - detecting the amount and type of light in different parts of a scene Feature detection - putting together information about the local features within a part of the image (lines, corners, simple shapes etc.) Object identification - putting together information about the combinations of features that identify an object - Each neurone has a receptive field - the receptive field is the 'window' of the visual field that the neurone is analysing. Only stimuli presented within the receptive field can change the electrical activity of that neurone. - Each stage of processing, say the 2nd stage of processing will add the information of the many neurones of the 1st stage, and then the 3rd stage will add the information of the many neurones from the 2nd stage and so on. So each stage gets a larger receptive field, and a receptive field that is more selective to what is happening. - The information seen is projected to the back of the eyes, then sent to the thalamus (LGN?) and from there to the visual cortex. The visual cortex is right at the back of the head. - Each side of your brain sees the opposite side of the visual field. However it should be noted that this does not mean that the everything from your right eye goes to your left brain, because both your left eye can also see this (eyes overlap). If you fixate on a point, everything from the left side of that point, goes to the right brain. - In the visual cortex, cells are no longer "spot readers". They code for the presence of orientated boundaries. - As seen above, orientation-selective responses can be generated by the convergence of inputs from several cells with non-oriented receptive fields, which are located along a line in space. - Within a small region of the primary visual cortex, there are neurones that are selective for every type of orientation. They all have the same receptive field (all covering the

2 same window), but they each see it from a different perspective (they detect whether their preferred orientation is present). - Feature extraction begins in the primary visual cortex (V1) - In the primate brain, different hierarchical levels of visual processing are arranged from the back of the brain (primary visual area, V1) to the front - V4 inherits the orientation selectivity from V1 but they become more explicitly selective to the SHAPE of the object. - Area V4 is an early stage of the shape and colour processing pathway (ventral stream) Cells in V4 are selective to parameters such as length and width of an object 2. Give examples of how the information becomes integrated in specific ways as the different hierarchical levels of the visual system are considered. - Once you reach the visual cortex, cells become more orientation-selective. That is, they only respond to an object that is of a specific orientation. - For example, if you look at a small region of the primary visual cortex (V1), there would be a group of neurons, each of which have the same receptive field but see it from a different perspective where they only respond if their preferred orientation is present. - As you move up the hierarchical stages, neurones become even more selective. For example, V4 inherits the orientation selectivity from V1, however becomes more explicitly selective to the SHAPE of the object. 3. The following statements are all correct, with the exception of: a) Neurones in the lateral geniculate nucleus (LGN) and the retina have very small receptive fields, which basically "read" the amount of light and the wavelength composition in a small "spot" of the visual field b) One of the first steps in creating a neural representation features such as the orientation of boundaries between surfaces reflecting different amounts of light. This process is accomplished for the first time in area V1 c) Different cells in a same visual area have receptive fields covering different sectors of the visual field. Together, they form a complete "map" of the visual field in the brain, which is systematically organised so that adjacent cells have partially overlapping receptive fields. d) In the human and monkey brain, areas corresponding to different hierarchical levels are organised in an anatomical sequence, from the frontal lobe backwards, so that the areas forming the highest levels (inferior temporal cortex) are located in the caudal (back) portion of the visual cortex. e) Cells in area V4, which is an intermediate level of the ventral visual cortex, are selective to both the length and width of an object, in addition to its orientation and colour.

3 - D) is incorrect. The hierarchical levels are actually organised from the occipital pole (V1, right at the back of the brain) forwards, so that the inferior temporal area is located in the most rostral part of the visual cortex.

4 Lecture 3: Cellular organisation and biophysical properties of the cerebral cortex 1. Distinguish the main types of neurones that form the cerebral cortex in terms of morphology, distribution in layers, and action (excitatory and inhibitory) - There are two main types of neurones that form the cerebral cortex. These are: Pyramidal cells Stellate cells - Pyramidal cells: Are the main type of cortical excitatory neurone They are the only cells that project long axons to other brain areas. However, most pyramidal cells project locally, forming intrinsic connections. They have a long apical dendrite with multiple branches, which projects towards the pia mater, as well as a complex basal dendritic tree The axons emerge towards the white matter Small pyramidal cells are found mostly in layers 2 and 3 of the cerebral cortex Large pyramidal cells are found mostly in layers 5 and 6 (layers 5 and 6 generally send information back to the thalamus) - Stellate cells: There are SMOOTH stellate cells and SPINY stellate cells and are only INTRINSIC (compared to pyramidal which is both). SMOOTH stellate cells are cortical INHIBITORY interneurons that act by modulating the activity of pyramidal cells. They are found in ALL cortical layers and use GABA as a neurotransmitter SPINY stellate cells are small cortical EXCITATORY interneurons which receive the bulk of thalamic afferents and relay these inputs to pyramidal cells. They are found primarily in layer 4 of the cortex as GRANULAR CELLS (layer 4 = granular layer) 2. Explain how vertical interactions across layers give rise to a columnar organisation 1. The main excitatory inputs arrive in the granular layer (layer 4) 2. This information is processed and integrated in the supragranular layers (2 and 3) 3. The processed data are relayed to other cortical areas (feedforward) 4. This data is also relayed to the infragranular layers (5 and 6), which then send feedback projections to "earlier" areas or to the thalamus

5 5. The feedback also modulates the processing in the granular and supragranular layers. - These interactions give rise to FUNCTIONAL COLUMNS, running from layers 1 to 6. - Every cell in a column shares some functional properties