Neuroscience you can sink your teeth into

CSGnet Archive CSG2010
1

[From Bruce Abbott (2010.92.27.1255
EST)]

A candidate for a position in
our Department of Psychology provided the following graphic, which shows the
rate of firing of two individual neurons located in the dorsal tegmental nuclei
of a rat. (I’m not exactly sure where in the brain it’s located,
but it’s apparently somewhere within the limbic area.) About 75% of
the neurons in this structure respond to angular head velocity (rotation) in
the horizontal plane. The rate of firing of some neurons increases with angular
head velocity in the same way whether the rotation is clockwise or
counterclockwise (left graph). Others change their firing rates in one
direction for clockwise rotation and in the other direction for
counterclockwise rotation (right graph). The data are from an investigation of
the neural pathways that contribute to a sense of “head direction”
(the direction in which the head is pointing within a familiar environment).
Single neurons have been identified that selectively fire a relatively high
rates whenever the rat’s head faces a particular direction. Included in
the candidate’s talk was a movie in which you could watch the rat moving
about in an arena and hear bursts of what sound like loud static whenever the
monitored cell became active. In the movie, this happened whenever the head
pointed at about the 8 o’clock position, as seen from above (with 12 o’clock
at the top of the frame). Other cells responded whenever the rat’s head
pointed at a particular location, regardless of the direction from which the
rat approached that location. The reported research is being conducted at Dartmouth.

Bruce A.

image0015.png

2

[From Rick Marken (2010.02.27.1005)]

Bruce Abbott (2010.92.27.1255 EST)--

A candidate for a position in our Department of Psychology provided the following
graphic, which shows the rate of firing of two individual neurons located in the dorsal
tegmental nuclei of a rat.

Not only can I sink my teeth into it; it turns my head. This is the
kind of neuroscience I like. I say hire the guy (or gal)!

Best

Rick

···

--
Richard S. Marken PhD
rsmarken@gmail.com
www.mindreadings.com

3

[From Bill Powers (2010.02.27.1208 MST)]

Bruce Abbott (2010.92.27.1255 EST) --

BA: ... The rate of firing of some neurons increases with angular head velocity in the same way whether the rotation is clockwise or counterclockwise (left graph). Others change their firing rates in one direction for clockwise rotation and in the other direction for counterclockwise rotation (right graph).

This is good info, but incomplete. The v-shaped curve is showing (approximately) the absolute value of angular velocity plus a constant, and the other shows angular velocity with sign, plus a constant. The negative-going peak in both graphs near zero velocity may show that they are related (that is, that somewhere previous in the neural net, the speed information is being used in computing the angular velocity information).

The focus on the neurons where these effects are measured implies that the computations are taking place in those neurons, rather then prior to them in a neural network of some kind. To see which is the case, we need to see graphs of the rates of firing of the signals coming into the dendrites of these neurons. We would need to see that anyway, to characterize the input-output characteristics of the neurons, but if the neuron is just summing a lot of input impulses, we would need to go upstream to find the places where the rate signals are actually being computed.

It doesn't seem likely that the neurons are responding to an absolute direction in space. How would the rat know the orientation of the cylinder relative to north, or the galactic center, or the First Point of Ares? There has to be something in the apparatus that is being seen -- for example, the transition from white to black in the wall of the cylinder that's about in the direction at right angles to the 8:00 direction. Rats are said to have no fovea, and to see almost a whole hemisphere on each side, so that white-black line would be about in the center of the field of vision of the right eye, perhaps a bit forward. The overlap of left and right fields that allows binocular vision is only about 70 degrees, or 35 degrees left and right.

It's interesting that these perceptual input functions are located in the amygdala, which is usually mentioned only in connection with emotions. These data seem to show that ordinary perceptual computations are performed here, with no special connection to emotions. I assume that the measured signals are on their way upward in the hierarchy, not downward.

The angular velocity plus constant arrangement has been mentioned by Henry Yin as a widespread feature of the brain: "The most striking design in the basal ganglia is the following: cortex-->(+) quiet striatum cell -->(-) tonically active nigra cell (say 20 Hz in the absence of input)-->(-) tonically active thalamus/brainstem cell (say also 20 hz in the absence of input. This disinhibitory circuit is very famous and characterizes the entire brain."

I said the following, and drew this diagram, in reply:

···

==========================================================================
A signal from the cortex enters a striatum cell, which generates a signal that enters a "phase splitter" in the nigra cell which creates two signals equal and opposite to each other relative to their neutral state. Would I be correct in guessing that the first tonically active cell sends outputs not only to the input of the next tonically active cell, but also past it toward the same general destination as the second cell's output reaches?

cortex --------> (+) striatum --->(-) Nigra1 ------> (-)Nigra2 ---
                                                > >
                                                v v
                                            neg output pos output

One increases as the other decreases. This could be useful in activating antagonist pairs of spinal control systems; e.g. triceps and biceps or similar bidirectional control systems at somewhat higher levels. I hadn't thought of this way of doing bidirectional control, but clearly it would work if somehow the succeeding circuits could use the mean frequency as the zero point.

Best,

Bill P.

4

[From Bruce Abbott (2010.02.27.1850 EST)]

Bill Powers (2010.02.27.1208 MST) --

Bruce Abbott (2010.92.27.1255 EST)

BA: ... The rate of firing of some neurons increases with angular
head velocity in the same way whether the rotation is clockwise or
counterclockwise (left graph). Others change their firing rates in
one direction for clockwise rotation and in the other direction for
counterclockwise rotation (right graph).

BP: This is good info, but incomplete. The v-shaped curve is showing
(approximately) the absolute value of angular velocity plus a
constant, and the other shows angular velocity with sign, plus a
constant. The negative-going peak in both graphs near zero velocity
may show that they are related (that is, that somewhere previous in
the neural net, the speed information is being used in computing the
angular velocity information).

BP: The focus on the neurons where these effects are measured implies
that the computations are taking place in those neurons, rather then
prior to them in a neural network of some kind. To see which is the
case, we need to see graphs of the rates of firing of the signals
coming into the dendrites of these neurons. We would need to see that
anyway, to characterize the input-output characteristics of the
neurons, but if the neuron is just summing a lot of input impulses,
we would need to go upstream to find the places where the rate
signals are actually being computed.

This is all new to me, but it appears that researchers have investigated a
number of nuclei that have connections with the dorsal tegmental nucleus and
have been trying to understand how the various parts of the network
contribute. For example, neurons that respond selectively to head direction
are found first in the lateral mammillary nuclei (LMN), which has reciprocal
connections with the dorsal tegmental nuclei (DTN). These signals are
further represented in a number of other structures "upstream."

BP: It doesn't seem likely that the neurons are responding to an absolute
direction in space. How would the rat know the orientation of the
cylinder relative to north, or the galactic center, or the First
Point of Ares? There has to be something in the apparatus that is
being seen -- for example, the transition from white to black in the
wall of the cylinder that's about in the direction at right angles to
the 8:00 direction. Rats are said to have no fovea, and to see almost
a whole hemisphere on each side, so that white-black line would be
about in the center of the field of vision of the right eye, perhaps
a bit forward. The overlap of left and right fields that allows
binocular vision is only about 70 degrees, or 35 degrees left and right.

In these experiments the rat was placed in a circular arena that had a white
segment on a portion of the wall. Apparently the animal learns the position
of this segment and the directional signals are relative to this. Some of
the experiments tested the effect of subsequently moving the position of the
white segment and found an effect, but from the talk I heard I seem to
remember that this rotation also changed the directional signal, but not by
as much as the rotation, possibly because the top of the arena permits some
view of the lab surroundings.

BP: It's interesting that these perceptual input functions are located in
the amygdala, which is usually mentioned only in connection with
emotions. These data seem to show that ordinary perceptual
computations are performed here, with no special connection to
emotions. I assume that the measured signals are on their way upward
in the hierarchy, not downward.

I don't believe that I mentioned the amygdala although I did say that
thought that these structures were in the limbic system. However, I've found
a mouse brain atlas that locates the DTN along the margins of the fourth
ventricle at the base of the midbrain, which would place it near where
auditory/vestibular signals enter the brain from the inner ear. Directional
signals appear in the mammilary bodies, which link via the fornix to the
hippocampus, so to that extent limbic structures are involved. The
hippocampus is involved in the laying down of certain kinds of memories.

Here's the URL for the mouse-brain atlas, sagittal view:

Place your cursor where the cerebellum (yellow) connects to the brainstem,
then right-click and select "zoom to this point" from the menu to get a
clear view of the position of the DTM.

If you do a Google search for dorsal tegmental nucleus, you will turn up a
variety of papers about directional neurons, including the one from which
the data were taken from the graph I presented (Bassett & Taube, 2001). The
latter is a pdf file that you can save.

Bruce A.