Visual Dysfunction, Objects, Size, and Shape Flashcards
issues within dyslexia
issues with reading, spelling, and event sequencing
what are issues in dyslexia due to?
due to impairments with the Magnocellular (dorsal) part of the visual system (involved in timing and rhythm via MT and STS).
cornelissen (1995), dyslexia, and motion coherence
found dyslexic people struggle with motion coherence - require a 3% increase in dots vs controls in order to perceive direction
what can explain the origin of dyslexia?
cerebellar deficits, as the Magnocellular system terminates in the Cerebellum
what can lead to impaired temporal processing?
issues with Magnocellular systems and cerebellar functioning
what is schizophrenia characterised by?
disorganised thinking and speech, poor memory, auditory hallucinations, and poor emotional and social function.
they also struggle to detect motion
what does autism impact?
social awareness and involves heightened sensory awareness, with an excessive attention to detail and change
autism and motion coherence
show higher motion coherence thresholds than typical children, revealing an impaired ability to detect coherent motion.
how can the associations between dyslexia, schizophrenia, and autism be explained?
possible that the dorsal system (includes STS) is involved in processing motion, event timing, and social stimuli, which are all disrupted in these conditions vs. biologically fragile dorsal pathway.
what is photosensitive epilepsy?
PE is a condition where seizures are triggered by temporal frequencies, e.g., stroboscopic flashing at 15HZ, and visual frequencies, e.g., 12pt line separation.
demographic factors and explanation of PE
experienced by 4% of the population due to increased excitability in the visual system.
EEG traces show the onset of a flashed stimulus causes widespread increase in electrical activity during seizures – highlighting the role of cortical excitation in PE.
what are migraines?
are severe prolonged headaches accompanied by nausea, photophobia and phonophobia.
similarity between PE and migraines
visual triggers for migraines may be similar to those for PE, suggesting a common mechanism.
what are migraines characterised by?
excessive excitability in the cortex and failure to inhibit excessive inhibition.
this leads to neuron fatigue and dilated blood vessels to avoid reduced oxygen, resulting in the later headache.
what can migraines be preceded by?
can be preceded by auras – ‘hole’ in the fovea which spreads to the periphery in a striped pattern.
what is visual stress (meares-irlen syndrome)?
shares symptoms with these conditions, and affects 20% of the population. They struggle to read printed text and see coloured blotches appearing in the text.
how is visual stress alleviated?
alleviated by using coloured overlays for paper and wearing precision-tinted lenses.
filters are successful by preventing excitation spread from local areas to other areas of the cortex
huang (2011) and prescribed lenses
FMRI showed reduced cortical activity in V3 (processes motion) and V4 (processes colour) when wearing prescribed lenses
for and against visual stress
long history of controversy and anecdotal evidence created skepticism for the condition.
broad symptoms are characteristic of many other conditions, which have not been screened for.
what is lateral occipital cortex (LOC) involved in?
combining shapes into whole objects
posterior LOC - detects parts of objects
anterior LOC - whole object recognition
classic ‘bottom-up’ view of vision
vision starts on the surface of the retina and works via increasing complex stages in even higher brain areas, towards some behavioural goal
how does the bottom-up view work for object recognition?
segment them from the background, identify contours, build contours into object outlines, and then recognise objects.
what is figure-ground segmentation?
involves separating objects from their background before they can be recognised. Contrast-sensitive neurons in each sub-system are responsible for this
what is responsible for figure-ground segmentation?
contrast-sensitive neurons in each sub-system are responsible for this
- RGC/LGN cells find object edges
- Orientation-contrast cells – texture-segmentation
- Motion-contrast cells – figure-ground for motion
- Disparity neurons – direct-object segmentation
what do illusory contours demonstrate?
the importance of top-down influences to fill in missing information, by perceiving a contour where no line exists at around 8m for static displays.
how do illusory contours occur?
occurs via the receptive fields of V2 cells, which detect the presence of the illusion of an object.
what are V2 receptive fields constructed from?
output of V1 hypercomplex cells, which detect line endings.
receptive fields at line endings will respond more strongly, which creates the perception of contour.
what is contour integration?
ability to link short sections of a contour to form a longer contour, to create the impression of a whole object.
when and how does contour integration occur?
occurs due to the long-range excitatory connections between cells with the same orientation, but different positions.
takes place in area V1 and develops around 2y.
role of form cells in V4
transform local contour information into coherent object perceptions
by selectively responding to contour features and integrating this information into representations of objects
what are geons?
there are 36 geons (geometric axons) that represent fundamental shapes to construct objects
what must occur between V4 and LOC?
the ability to construct an object
how is LOC involved in shape recognition?
by integrating information across different areas, rather than specific object identification.
what do object classification tasks suggest about vision?
vision is much easier via top-down view –
as higher-order information influences lower-down information by spotlight of attention to speed up recognition
how does top-down information refine the guesses of the visual system?
compares predictions to higher-order information
what does redundancy show?
learning is easier for predictable words and text (top-down information)
what top-down connections (_______ and ______) can guide attention to speed up recognition?
knowledge and expectations
shepard (2009) table top illusion
3D shape perception is important for the table top illusion,
as the retina sees the 2D line drawings as different, this means the brain assumes the 3D shapes must also be different
effect of autism on the table top illusion
less susceptible to the illusion, as they rely more on sensory experience and less on top-down influences
they believe the tables are the same size
how can size constancy be distorted?
beuchet chair and ames room
- retinal image is not scaled to the perceived distance
what happens when the perceived distance is too small?
we perceive things smaller than they really are
what does forced perspective employ?
optical illusions to make objects appear a different distance than reality.
