Lecture 7-January 28th (from retina to brain)Hypothesis for why really bright light hurts: When you’re exposed to very bright light, you have a reflex where your iris constricts which reduces the size of the pupil. Maybe if this reflex is too strong the TRG nerve feels that and perceives it as pain. Continuation of visual Acuity: Retinal ganglion cells and stripes: Ganglion cells with centre surround receptive fields respond preferentially to certain frequencies.In the image below the plus is the centre on and the minus is surround off. It shows how the cell preferentially fires to different frequencies. If the full on centre is exposed to light and the off surround is in the dark the cell will fire a lot of action potentials. The phase is also important: the phase refers to the grating’s position within a receptive field. If you look at the image below you see that the grating is moving from left to right. In a) it is perfect for that cell, the light is in the centre, the dark is in the surround. But if you move to b) the centre is a bit in the dark…c) the centre is all in the dark (so cell will have a negative response, the firing rate of the cell will go below the spontaneous firing rate of the cell).
Contrast: intensity difference between lightest and darkest portions of the patch. Contrast sensitivity function: visibility of a pattern as a function of spatial frequency and contrast. On the x axis we have the spatial frequency from very low to very high, and on the y axis is contrast sensitivity (how good we are at perceiving those frequencies as a function of contrast). The y axis is the contrast from very low to very high. Even if we have very low contrast we will be able to perceive those frequencies. If we go to higher and higher frequencies, contrast sensitivity decreases up to a certain point. We are moresensitive to frequencies in that middle range (1-10hz).