FAQ’s Sensory Physiology

Lecture 1: The Eye

Q: Function of Antagonist Surround-- how do you know which ganglion cells activities increase or decrease at the edge? On the web one ganglion cell increasing activity at the edge and one decreases, but how can you tell?

A: The receptive fields of two on center ganglion cells are shown. In the previous diagram we saw that if all the receptive field were in the light or the dark there is little change in activity. In the next diagram, the one on the bottom decreases its activity. This is because only the only part in the light is part of the inhibitory surround. The one on the top increases its activity. This is because only the only part not in the light is part of the inhibitory surround. Thus there is less inhibition of this cell.

Q: The figure you showed with the yellow dot before adaptation and then after adaptation.  With the yellow dot becoming in the zero velocity range.  Please explain the last statement of our notes.  "The result is a percept of motion in the opposite direction."

A: If, before adaptation, you were to activate these neurons in your MT, you would sense a slow motion say to the left. After adaptation a stationary stimulus now actives them.

Lecture 2: The Visual Cortex

Q: Is there a calcarine sulcus on both sides of the brain?

A: You should be able to answer this yourself. Hint, where are things in the left and right visual field represented?

Q: Does the length of the line change firing rates for simple and complex cells or does firing only change with orientation of the cells?

A: The cell would fire more for a line that fills its receptive field than for a shorter line. But unlike end stopped cells, it will not fire less for a very long line that extends beyond the receptive field.

Q: In problem 10, where the optic chiasm is severed, is the reason why the difference in what you see, as compared to an optic chiasm that’s severed for objects that are far, is because when you focus on near objects your eyes are somewhat crossed. By telling us the point of the plate that we are staring at, does this allow us to determine what will and will not be visible?. Essentially anything farther than the point of focus will be visible since it will project to the temporal side of each eye where the nerves are intact but anything closer will not be visible since the optic chiasm is damaged.

Not quite right. A anything farther than the point of focus will be Invisible since it will project to the NASAL side of each eye where the optic chiasm is damaged but anything closer will be visible since the nerves are undamaged. See top of lecture 2 page 13.

Lecture 3: Visual Perception of Objects

Q: When will both the what and where stream be activated at the same time? A: When the task involves both perception and action. For example if you had to pick up a pencil off a crowded desk you require the “what” stream to rcognize the pencil and the “where” stream to direction your arm to it.

Q: If an object, say an animal, is never seen before how are cells in the IT respond? A: Good question. I’m not sure, but perhaps they don’t respond. But LOC cell would because they respond to any object.

Lecture 4: Visual Perception of Motion

Q: On page 13 of the notes there is a diagram of a white square moving from the center of 4 circles and then into a segment of lines?  Is this motion detected by the MT or the what stream alone? A: It is detected by MT through the What Stream.

Q: The terms retinal slip and corollary discharge aren't very clear to me. Please explain.  I also don't understand the diagram on page 11 with the retinal slip and the corollary discharge acting to determine the perception of motion.  Please explain. A: Retinal slip. Is the visual motion that the eye sees.

In the figure, corollary discharge a copy of the command sent to the eye muscles to rotate the eye.

Lecture 5: Association Cortex

Q: I am confused about PTO lesion. Lecture note 5-10 says that in right PTO lesion, the left side of the face is neglected no matter the face is shown to the right or left of fixation.

But, in the case of problems at the end of lecture5 such as problem1, the solution is different. The person who has a right PTO lesion neglects the left side of the room. I thought the person neglects the left side of objects in both the right side and left side of the room.

This concept is quite confusing. Can you explain this again for me?A: Yes I guess this is not well explained. It is a question of where the frame is drawn i.e. what you are attending to. If you are attending an object, then it is the left side of the object. If you are attending a room, then the frame is the room, and you neglect the left side of the room.

I hope this helps.

Q: I am confused with question #4 in lecture 5. How is it possible that a PTO patient neglects the upper part of a watch after it is turned? I thought he will neglect the new left part.A: The watch is treated as an object (i.e. a frame drawn around the object) with a left side, top side, right side and bottom side. When the watch and its frame is turned, the left side is now on the top. Your answer would be correct if it were represented in an egocentric frame.

Lecture 6: Streams for Visually Guided Actions

Q: On page 6-9 of the notes, it explains what occurs when visual stimuli are flashed in sequence and the instruction is to saccade from one stimuli to the other. In the upper diagram representing immediately after the visual stimuli are presented, why is the location of activity in FEF still in the area of the midline and not at point A or point B or somewhere between?. A: The foveal region of FEF fires between saccades. Its firing maintains fixation. The diagrams show the activity of FEF before and after an saccade. The only time other regions of FEF fire is during a saccade.

Lecture 7: Touch

Lecture 8: Muscle Sense

Lecture 9: Hearing

Q: (Re Diagram on 9-6). In other words, does high frequency in a clarinet make a low sound, and vice versa? A: Each note also contains overtones; that is the original note, plus twice the frequency, and 4 times the frequency etc. How much of each varies from instrument to instrument and voice form voice.

Lecture 10: Balance

Q: Some books say that VOR is a tri-synaptic reflex, but the problem set says has 4 synapses? What do we go by?A: The text books are wrong. You work it out. Count them. Some text books forget the synapse between the hair cell and the 8^th nerve.

Lecture 11: Eye Movement

Lecture 12: Memory

Q: In Lecture 12, its says that visual cortex can be consolidated afterthe critical period. Doesn't this contradict Lecture 1 that says the visual system is plastic early in life i.e. only during the critical period?A: The critical period is an early phase in the development of the cortex in which neral connections are very plastic. A cataract in one eye before the age of 1 will produce a profound lose of binocular cells. A cataract at that develops late in life will have little lasting effect after it is removed. But even in old age, the visual cortex can be trained, with repetitive practice, to detect smaller and smaller breaks in a line. That suggests that it retains some plasticity.