1. ANATOMY AND PHYSIOLOGY OF THE CENTRAL NERVOUS SYSTEM
37
no orderly sequence of the different types of unit responses in the next
two tectal layers [B and C, alternating layers of fibers and cells of
Ariens Kappers et al. (1960)l. After passing through the upper three
layers (A, B, and C ) of the tectum the electrode entered a zone of
sustained on and off responses [layer D, within central gray of Ariens
Kappers et al. (1960)l. Units in this zone discharged actively in the
light and some were continuously active in the dark. This was an extremely precise zone since a 20-p excursion of the tip of the electrode
resulted in the loss of units with this response. Recordings made from
layer F [periventricular gray of Ariens Kappcrs et a2. (1960)l from the
optic tectum demonstrated units which were on, off, and on-off and
were presumably units whose axons comprise portions of the tectal
efferent system. The responses of these units were similar to those units
which were found in the most superficial layers of the optic tectum. The
three types of unit responses were found at various depths within the
tectum and were also recorded from nerve fibers in the optic chiasma.
On units with inhibitory surrounds were common in the superficial layers
(A, B, and C ) of the optic tectum and in layer F but only one such unit
was found in layer D. The diameter of the receptive field center of the
tectal neurons varied in different units when the visual angle of the
stimulus subtended from 10" to 30" (Fig. 5 ) . Directionally sensitive units
were located when the stimulus was moved parallel to the horizontal
meridian of the visual field. Directionally sensitive units could be detected more frequently when stimuli were moved from the temporal to
nasal field rather than for movements in the opposite direction (Jacobson
and Gaze, 1964; Schwassman and Kruger, 1965).
These electrophysiological results indicate that visual units are better
equipped to detect horizontal rather than vertical movement within the
visual field. This has been demonstrated behaviorally by Mackintosh and
Sutherland ( 1963). This visual phenomenon has been correlated with
rheotactic behavior (orientation of the longitudinal axis of the body in
moving water) since the animal is visually better equipped to detect
movement in the temporal-nasal direction ( Cronly-Dillon, 1961, 1964 ) .
On-off units with well-demarcated response zones were the most common type of unit found in all the fiber layers except layer D. These units
were also directionally selective in their response. In some units a 2"
spot of light moved through the field in a temporal-nasal direction resulted in a discharge, but reversal of this pattern of movement did not
result in a response. The receptive field of on-off units increased in size
following dark adaptation. On units were recorded from all the fiber
layers of the tectum but were found most frequently in layer D in which
on-off units or on units with inhibitory surrounds were rarely found. The
37
no orderly sequence of the different types of unit responses in the next
two tectal layers [B and C, alternating layers of fibers and cells of
Ariens Kappers et al. (1960)l. After passing through the upper three
layers (A, B, and C ) of the tectum the electrode entered a zone of
sustained on and off responses [layer D, within central gray of Ariens
Kappers et al. (1960)l. Units in this zone discharged actively in the
light and some were continuously active in the dark. This was an extremely precise zone since a 20-p excursion of the tip of the electrode
resulted in the loss of units with this response. Recordings made from
layer F [periventricular gray of Ariens Kappcrs et a2. (1960)l from the
optic tectum demonstrated units which were on, off, and on-off and
were presumably units whose axons comprise portions of the tectal
efferent system. The responses of these units were similar to those units
which were found in the most superficial layers of the optic tectum. The
three types of unit responses were found at various depths within the
tectum and were also recorded from nerve fibers in the optic chiasma.
On units with inhibitory surrounds were common in the superficial layers
(A, B, and C ) of the optic tectum and in layer F but only one such unit
was found in layer D. The diameter of the receptive field center of the
tectal neurons varied in different units when the visual angle of the
stimulus subtended from 10" to 30" (Fig. 5 ) . Directionally sensitive units
were located when the stimulus was moved parallel to the horizontal
meridian of the visual field. Directionally sensitive units could be detected more frequently when stimuli were moved from the temporal to
nasal field rather than for movements in the opposite direction (Jacobson
and Gaze, 1964; Schwassman and Kruger, 1965).
These electrophysiological results indicate that visual units are better
equipped to detect horizontal rather than vertical movement within the
visual field. This has been demonstrated behaviorally by Mackintosh and
Sutherland ( 1963). This visual phenomenon has been correlated with
rheotactic behavior (orientation of the longitudinal axis of the body in
moving water) since the animal is visually better equipped to detect
movement in the temporal-nasal direction ( Cronly-Dillon, 1961, 1964 ) .
On-off units with well-demarcated response zones were the most common type of unit found in all the fiber layers except layer D. These units
were also directionally selective in their response. In some units a 2"
spot of light moved through the field in a temporal-nasal direction resulted in a discharge, but reversal of this pattern of movement did not
result in a response. The receptive field of on-off units increased in size
following dark adaptation. On units were recorded from all the fiber
layers of the tectum but were found most frequently in layer D in which
on-off units or on units with inhibitory surrounds were rarely found. The