Unilateral lesions of the dorsal striatum in rats disrupt responding in egocentric space

Abstract

Rats were trained in a specially designed, multichoice operant chamber on a visual choice reaction time task designed to assess performance on each side of the rat's body. The task required animals to sustain a nose poke in a central hole, until a brief light stimulus was presented in either of two holes that were located on the same side of the box. Once the rats were trained to perform the task to both sides independently they received unilateral injections of quinolinic acid into the dorsal striatum. Postoperatively, lesioned animals were impaired when performing the task on the side contralateral to the lesion. The time taken to initiate contralateral responses was increased. Contralateral responses were also exclusively biased toward the nearer of the two response locations, regardless of the location of the stimulus. This was interpreted as a specific impairment in generating responses in contralateral space. In contrast, no comparable deficit was seen when the animals performed the task on the side ipsilateral to the lesion. Additional postoperative challenges, in which response options were presented bilaterally, showed this response deficit to be defined in egocentric coordinates, with the severest response deficits for the most contralateral locations.

Fig. 1.

Task requirements when response holes are configured to the left (A) and right (B). The rat must sustain a nose poke in the center hole. After a variable period (delay, 50, 250, 450, or 650 msec) a brief light flash appeared in one of the two holes to the side of the rat. The rat then had to withdraw its nose from the center and poke its nose into the same hole in which the light had appeared to obtain food reward.

Fig. 2.

Photomicrographs illustrating the effects of unilateral injections into the dorsal striatum of buffer solution (A, C) and 0.09 m quinolinic acid (B, D). The lesion produced extensive neuronal loss, as shown by the Nissl stain (B). The acetylcholinesterase-stained section exhibits a marked decrease in enzyme activity over a similar area (D). Scale bar, 1 mm.

Fig. 3.

Response bias to the near hole, for both groups when performing the task to the side ipsilateral (IPSI) or contralateral (CONTRA) to the lesion [% NEAR BIAS = (all correct and incorrect responses to the near hole)/(correct and incorrect responses to both holes)]. Before the lesion, both groups displayed a mild bias toward the near hole. Postoperatively, lesioned animals directed virtually all contralateral responses toward the nearer of the two holes. They also showed a less marked increase in near hole bias on the ipsilateral side. Bars indicate the SEM of individual animal means.

Fig. 4.

Preoperative and postoperative reaction times for correct responses, plotted as a function of delay, for each group. The lesion group showed no postoperative increase in reaction time when responding ipsilaterally. However, it did show an increase in initiating responses, which were contralateral to the side of the lesion, at each delay.

Fig. 5.

Preoperative and postoperative movement times for correct responses, plotted as a function of delay, for each group. The lesion group showed no lateralized impairments after surgery, although it took slightly longer to complete far hole responses.

Fig. 6.

Near hole response bias for ipsilateral (IPSI) and contralateral (CONTRA) configurations for postoperative sessions when trials consisting of both stimuli (extinction) and no stimuli were presented. On the contralateral side, lesioned animals still responded virtually exclusively to the far hole in all stimulus conditions. Ipsilaterally, the lesion group performed as shams. BASE, Either light;DBLE, both lights; NONE, no lights presented. The hole configurations are shown above each graph, as they would appear for an animal lesioned on theleft.

Fig. 7.

Ipsilateral response bias for the four bilateral hole configurations, for postoperative sessions when trials consisting of both stimuli (extinction) and no stimuli were presented. Lesioned animals tended to bias their responding to the ipsilateral (IPSI) side when the far contralateral (CONTRA) hole was one of the holes used. The extinction task also tended to increase the ipsilateral bias in lesioned animals. However, lesioned animals showed no bias in those conditions in which no stimulus was presented. BASE, Either light;DBLE, both lights; NONE, no lights presented [% IPSILATERAL BIAS = (all correct and incorrect responses to the ipsilateral hole)/(correct and incorrect responses to both holes)]. The hole configurations are shownabove each graph, as they would appear for an animal lesioned on the left.