@5 on Thursday, 04 May, 2000
gratings of decreasing periodicity � fall-off in contrast sensitivity at very high and very low frequency. between = window of visibility. at the very highest frequency, the spaces seem to get bigger again
contrast � white board rubber against blackboard � fraction of reflected light (remains the same under different overall luminosities, giving similar contrast under different light conditions)
mosaic of receptors in the Macaque monkey
focussing
pupil size � decrease in pupil size �/span> � contrast sensitivity, because cuts out spherical aberration from the optical system
dots with overlaid gratings (sinosoidal function) �/span> interference (moir�pattern
failure of the Nyquist sampling limit
why don�t we see such high frequency usually, and see everything in the moir�attern � because the optics/lens blurs the image so that it�s not high enough frequency to give rise to that pattern. the properties of the optics and the properties of the mosaic are closely matched
very narrow peak of cone density function
reflected in the cortex representation of the visual field � a lot of cortex is devoted to 5deg of the visual angle (the area of a fist held at arm�s length). we are foveal creatures, we need to point our eyes at something to see it at high resolution
Enroth-Cugell and Robson, 1966
X-cells � similar to P-cells
grating switched on, with a bright bar in the centre of the cell � high response, levelling slightly below
null phase � grating on: left half bright, right half dark � no response = linear spatial summation
Barlow + Mollen � wrong, suggests that this is due to the centre-surrounds � don�t need centre-surround antagonism to cancel out
cells � little contrast meters
random little grey + white bars flashing across a black background
reverse correlation � listen for when an AP occurs, then look back to when a stimulus occurred
green circle surrounded by doughnut of red, then mutates into red circle
after the stimulus, get a response; when you then take the stimulus away, you get an opposite response
bi-phasic response
dancing spots + gratings � totally different methods, give convergent responses
take a picture, and flash it across in different positions of the receptive field of the same neuron (in the LGN)
this isn�t a representation of what the brain sees � it�s a representation of what the cell does
the linearity of the firing rate of the cells is related to the fact that we can predict (from arbitrary stimuli) of how the cell is going to respond
criterion of linearity = powerful model
Henry Fox-Talbot
rustic barn etc. exhibition on the history of photography
in the fovea, P-cells = 95% of the ganglion cells
visual receptive fields touch, leaving no gaps
assert that you can associate particular functions with the magno and parvo layers of the LGN, asociated cortically with �what� and �where� system
magno are much more sensitive to contrast than the parvo
if you lesion an animal�s magno or parvo cellular layer (can do cleanly/selectively with hibertinic acid???)
parvos make up for their weakness in contrast sensitivity in numbers
isn�t really true that black and white high contrast goes through the magno � need to take into account population, not just individual cells� physiology
Merrigan & Maunsell � measured spatial acuity as a function of eccentricity. knocked out the parvo layers of the LGN, and noticeable supression of the acuity there
if knock out the parvo-cellular layers with acrilimide -