Figure 1.
Schematic diagram illustrating direct and indirect pathways connecting mossy fibers (MF) to GLIs.
Arrows indicate the directionality of the signal flow. Granule cells (GrC) receive glutamategic inputs from MFs and UBCs, and glycinergic/GABAergic inputs from Golgi cells (GoC). Unipolar Brush cells (UBC) receive a single mossy fiber input and glycinergic/GABAergic inputs from nearby Golgi cells. UBCs in turn establish glutamatergic synapses with granule cells and other UBCs. Golgi cells receive glutamatergic inputs from mossy fibers and parallel fibers, and glycinergic/GABAergic inputs from other Golgi cells. For simplicity we show only the classical major connections and interneurons of the granular layer. We also separate them between glutamatergic pathways (green) and glycinergic/GABAergic pathways (blue), although we acknowledge that other neurotransmitters, pathways, and interneurons exist. Axons and dendrites are represented by thick and thin traces respectively.
Figure 2.
(A) Raw data showing the response of an example mossy fiber to 5 deg and 20 deg leftward and rightward off-center fixation trials. The upper row shows horizontal eye position (negative values for leftward eye position and positive for rightward eye position), the middle raw spike data, and the bottom row instantaneous firing rate (IFR, black dots). Blue circles in the lower row represent the average firing rate following each saccade (200–700 ms). (B) Relationship between average firing rate and eye position using data from trials like those shown in A. (C) Best fitting line for each mossy fiber that could be well fit with F1, showing the relationship between eye position and firing rate. The eye position has been normalized to display the preferred direction of the neuron as positive. The average response profile across all mossy fibers is shown in red.
Table 1.
Types of GLIs categorized based on their response profile during the “Off-center fixation” task; mossy fibers (MF) are shown in the bottom row for comparison.
Figure 3.
Response of one GLI with a linear response profile (Group I).
(A) The upper row shows horizontal eye position, the middle raw spike data, and the bottom row instantaneous firing rate (IFR). Blue circles in the lower row represent the average firing rate following each saccade (200–700 ms). (B) Relationship between average firing rate and eye position using data from trials like those shown in A. The response of this example GLI was closely fit by a linear function F1 (VAF = 99.6%).
Figure 4.
Responses of example Group II (A–C) and III (D-G) GLIs.
(A, D) The upper row shows horizontal eye position, the middle raw spike data, and the bottom row instantaneous firing rate (IFR). Blue circles in the lower row represent the average firing rate following each saccade (200–700 ms). (B, E) Relationship between average firing rate and eye position using data from trials like those shown in A and D. Both cells were fit with the function F2. The corresponding fits are shown as green lines in B and E. (C, F) Response of the each example neuron during sequential saccades (5 deg). The upper row shows in black traces the eye position and in red traces the laser position; the lower row shows the instantaneous firing rate (IFR). (G) Response of the example Group III GLI shown in D–F during long eye fixations. The upper row shows in black traces the eye position and in red traces the laser position, the lower row shows the instantaneous firing rate (IFR).
Figure 5.
Responses of example Group IV (A–C) and V (D–F) GLIs.
This figure uses the same layout as Figure 4A–F. (A, D) The upper row shows horizontal eye position, the middle raw spike data, and the bottom row instantaneous firing rate (IFR). Blue circles in the lower row represent the average firing rate following each saccade (200–700 ms). (B, E) Relationship between average firing rate and eye position using data from trials like those shown in A and D. Both cells were fit with the function F3. The corresponding fits are shown as green lines in B and E. (C, F) Response of the each example neuron during sequential saccades (5 deg). The upper row shows in black traces the eye position and in red traces the laser position; the lower row shows the instantaneous firing rate (IFR).
Figure 6.
Modeling the responses of GLIs.
(A) Average response profile of mossy fibers, identical to the red line in Figure 2C, but shown as two different tuning curves with opposite directional preferences for illustration of the fitting approach. (B–E) Model fits of the response profiles of the example Group II (B), IV (C), III (D) and V (E) GLIs. The weights w1 and w2 used for the fits are indicated in blue and magenta. The offsets FR0 are indicated by black broken lines (in C, the offset is negative). (F) Distribution of VAFs produced by the model across the population of GLIs. (G) Scatter plots showing the weights obtained by model fitting. The example Group I–V GLIs shown in Figure 3–5 are plotted with black (group II–V) and gray (group I) borders and with roman numbers to the left indicating the GLI group they belong to. The asterisk indicates a group V GLI with a w2/w1 ratio near zero (the response profile of this GLI is shown in Figure S3). This cell was atypical among group IV and V GLIs because, unlike the rest of group IV and V GLIs whose responses were best fit by two slopes of opposite sign (i.e., direction), the slopes of the two lines that best fit the response of this cell had the same direction.
Figure 7.
Firing properties of the GLIs and mossy fibers.
(A) Average firing rate and CV2 of the various neuronal elements. GLIs (circles) are color-coded according to their response properties: ‘undecided’ in black, ‘excited’ in green, and ‘inhibited’ in red). (B–G) Attenuation of spike amplitude at high firing rates. Panels B, D and F show the raw trace (top) and IFR (bottom) of three example cells. Panel C, E and G show the normalized spike amplitude (1 corresponds to the median amplitude when ISI>50 ms) of GLIs and mossy fibers (G) as a function of interspike interval (ISI). The circles indicate the curves corresponding to the example cells in panels B, D and F. (H) Attenuation Factor (median spike amplitude when ISI>50 ms divided by the median spike amplitude when ISI<30 ms) of different GLI classes and of mossy fibers. (I) Relation between CV2 and Attenuation Factor in GLIs. The example cells in B, D and F are plotted with black (B and D) and gray (F) borders.