S1 LayerΒΆ

The S1 layer is the simple cell layer. It consists of orientation and spatial frequency (SF) tuned filters. The layer is configured by the <S1> parameter object.

Orientation and phase

The number of orientations and phases represented are set by the following parameters:

nori     4          # Number of orientations (evenly distributed in 180 deg)
nphase   2          # Number of phases (evenly distributed in 360 deg)
phase0  90.0  (deg) # Starting phase

The first line above specifies that there should be 4 orientations (thus, 0, 45, 90 and 135). The second line specified 2 phases. The third line specifies that the first phase value should be 90 deg, and thus the second phase value will be 270 deg, so that the values remain equally spaced within a 360 deg circular dimension.

Gabor function geometry and amplitude

The proportions of the Gabor filters are set relative to the Tile Size (specified further below) by the following scale factors:

gabor_sd_orth_f  9.0     #  SD_orth = TileSize / f
gabor_sd_para_f  6.0     #  SD_para = TileSize / f
gabor_sf_f       3.0     #       SF = f / TileSize
filter_norm_targ 0.0     # Target value for filter scaling: area of |f|
                         #   If 0.0, do not apply this normalization to the
                         #   S1 Gabor filters

The first line specifies that the standard deviation (SD) of the Gaussian orthogonal to the filter orientation will be 1/9 of the Tile Size. The second line specifies that the SD parallel to the filter orientation will be 1/6 of the Tile Size. (Thus, the Gabor filter will be longer along its bars than it will be wide, typically of a V1 receptive field.) The third line specifies that the spatial frequency (SF) of the sine component of the Gabor filter will be 3 divided by the tile size, i.e., there will be 3 cycles of the sinewave within the distance equal to the tile width.

The fourth line in the parameter list above specifies how the amplitude of the filters is set. If the value of “filter_norm_targ” is set to 0.0, the the filters will not be rescaled. If this paramter is set to a value greater than 0, then the filters will be rescaled so that the sum (over all x and y pixel positions) of the absolute value of the filter is equal to the specified positive constant.

Normalization

The outputs of the S1 units are subject to normalization that is controlled by the following parameters:

norm_type        1       # 0-none, 1-local stim squared
norm_eps         0.0001  # Added to denominator to avoid div by zero
norm_mask_sd_f   2.5     # Normalization mask radius = f * max(SD_o,SD_p)
                         #   E.g. rad 15 for tile_w 36 (given ..sd_para_f = 6)

Tiles

The set of S1 tiles determines the number and size of the Gabor filter channels. Tiles are specified by repeated use of the <tile> object. An example specification for a set of six tiles is as follows:

<tile>
   name 1
   size 32  (pix)  # Size of RF tile in pixels
</tile>
<tile>
   name 2
   size 36  (pix)  # Size of RF tile in pixels
</tile>

...

<tile>
   name 6
   size 60  (pix)  # Size of RF tile in pixels
</tile>

Each tile should be named with a number (here 1, 2, ... 6) and must have a “size” parameter that is set to an integer number of pixels. These size values are the width and height of the tile, and are referred to as Tile Size in the section above on Gabor filter geometry.