bipolar cells in light
light = photoreceptor is hyperpolarised = less glutamate onto bipolar cells = disinhibition =
- exites the ON bipolar cells that in turn excites the ON ganglion cells
- inhibits OFF bipolar cells = removes exitatory drive from ganglion cells
ganglion cell projections
20% projections to sup colliculus: Superior colliculus: responsive to sudden shifts in light
80% to lat. geniculate nucleus (thalamus) → V1
retina-geniculate-striate pathway
retina -> lat gen nucleus (thalamus) -> V1
Parvocellular vs Magnocellular cells
inhabit distinct layers of the LGN
- Parvocellular cells
- small cell bodies
- layers 3-6 of LGN
- respond to colour, fine detail, still or
slow-moving objects
- Magnocellular cells
- big cell bodies
- layers 1-2
- respond to objects in motion
processing in visual cortex
- hierarchical (V1 → V2 → V3…)
- but also shows concurrent processing (V1 ↔ V2 ↔ V3…)
V1
- V1 neurons have smallest receptive fields of any region of visual cortex (and thus highest resolution)
- responsive to colour, orientation, spatial frequency
- processes boarders, edges, contrast
- = contour enhancement/feature detection
colour perception/processing
V1, V2, V3 responsive to wavelength, but actual colour perception occurs at V4
object perception
- V1-3 does the basics of simple form analysis (orientation, spatial frequency, wavelengths)
- V4 adds perception of colour (i.e. conscious awareness of colour), and some complex contours
- neurons respond to curvature and orientation
- also involves on top-down processing
- don’t need all the info to make a picture, eg illusions
top down processing in object recognition
- e.g. optical illusions, can switch between what we ‘see’ based on the top-down perception we are imposing
- emphasis on pattern recognition
- automated recognition of patterns
- like when learning music, sport
- e.g. continuity, closure
- Gestalt psychology (German. lit. “form” or “pattern”) but reflects the idea that the whole is different from the sum of its parts.
- I.e. the brain creates a percept that is more than simply the sum of available sensory inputs
motion perception
V5 (aka MT)
- all neurons motion selective
- most direction selective too
- input is from V1 motion sensitive cells (for slower movement), or straight from thalamus to V5 for faster motion/survival responses
MTS and STS
- Medial Superior Temporal (MST)
- receives input from MT
- responds to complex motion
- neurons here have large receptive fields, tuned to respond to different speeds in different areas of visual field
- relative motion of scene and observer
- Superior temporal sulcus (STS)
- biological motion
- species specific
- social perception - recognition and communication (speech, facial and biological movement)
THE dress
theory: morning birds, more see more blue light, brain subtracts this out, left with the white and gold
- also how you subconsciously perceive the direction of the light, and the quality of it
- image entering our brains are the same, but differ in how we transform the image
double drift illusion
- see both as moving diagonally, despite one actually moving vert
- due to alt b and w shapes
- different responses to stimuli in V1, but same in PFC (judgement and decision making)
flash lag illusion
- 100ms processing time from retina to brain
- so brain predicts what it thinks is going to happen
- allows us to respond/get to the target quickly and accurately
Adam Hantman’s view on the roles of the brain and the sensory systems in generating conscious perception
- what we experience as consciousness is primarily the prediction, not the real-time feed
- Our brains like to predict as much as possible, then use our senses to course-correct when the predictions go wrong
