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Representative Publications

Horvitz, J.C., and Jacobs, B.L. Fundamentals of Behavioral Neuroscience textbook, Oxford University Press, in preparation.

Coffey, K.R., Barker, D.J., Ma, S., Root, D.H., Martinez, L., Horvitz, J.C., West, M.O. Effects of Varying Reinforcement Probability on Pavlovian Approach Behavior and Ultrasonic Vocalizations in Rats. Behavioural Brain Research, 237, 256-62, 2013.

Choi, W. Y., Campbell, C. M., Balsam, P. D., & Horvitz, J. C. Effects of cortical and striatal dopamine D1 receptor blockade on cued versus noncued behavioral responses. Behavioral Neuroscience, 125, 705-713, 2011

Ashby, F. G., Turner, B. O., & Horvitz, J. C. Cortical and basal ganglia contributions to habit learning and automaticity. Trends Cogn Sci, 14, 208-15, 2010.

Choi, W., Morvan, C., Balsam, P.D., Horvitz, J.C. Dopamine D1 and D2 antagonist effects on response likelihood and duration. Behavioral Neuroscience, 123, 1279-87, 2009.

Horvitz, J.C. Stimulus-response and response-outcome learning mechanisms in the striatum. Behavioral Brain Research, 199, 129-40, 2009. [PDF]

Wickens, J., Horvitz, J.C., Costa, R., Killcross, S. Dopaminergic Mechanisms in Actions and Habits, Journal of Neuroscience, 27, 8181-3, 2007.
A review of recent work at cellular and behavioral levels of analysis relevant to dopamine’s role in the learning and performance of goal-directed behaviors and S-R habits.

Horvitz, J.C., Choi, W., Morvan, C., Eyny, Y. Balsam, P.D. A “Good Parent” function of dopamine: transient modulation of learning and performance during early stages of training. Annals of the New York Academy of Sciences, 1104, 270-288, 2007.
The paper describes a theoretical perspective on dopamine’s role in the learning and performance of goal-directed behavior. Recent work in our laboratory suggests that dopamine plays a critical role in both learning and performance of behaviors during early stages of training, and that dopamine’s contribution to behavioral performance diminishes as the behavior becomes a well-acquired habit.

Rick, J.H., Horvitz, J.C., Balsam, P.D. The Effect of Dopamine Receptor Blockade on Behavioral Variability and Comparisons to Extinction, Behavioral Neuroscience, 120, 488-92, 2006.
It has long been held that reductions in dopamine transmission reduce the ‘reward’ value of reinforcers.Following dopamine receptor blockade, animals show reduced rates of operant responding similar to those seen in animals for whom reward delivery is omitted, i.e., animals show what appears to be behavioral ‘extinction’.  However, during extinction, animals show not only reduced operant response rates but also an increase in behavioral variability. This paper demonstrates that while dopamine receptor blockers reduce rates of reinforced responding, they do not produce the increase in response variability that characterizes extinction.

Horowitz, T.S., Choi, W., Horvitz, J.C., Cote, L.J., Mangels, J.A. Visual search deficits in Parkinson’s disease are attenuated by bottom-up target salience and top-down information, Neuropsychologia, 44, 1962-77, 2006.
Dopamine neuronal responses appear to play an important role in marking environmental stimuli as ‘salient’, i.e., promoting basal ganglia and frontal cortical processing of particular stimuli in the environment. This paper is a detailed examination of the attentional difficulties that arise in patients that have suffered a loss of nigrostriatal dopamine neurons.

Stern E.R., Horvitz, J.C., Cote, L.J., and Mangels, J.A.Maintenance of response readiness in patients with Parkinson's disease: evidence from a simple reaction time task, Neuropsychology, 19, 1, 54-65, 2005.
This paper examines aspects of motor difficulties in Parkinson's patients, a group that suffers loss of nigrostriatal dopamine neurons.

Choi, W., Balsam, P.D., Horvitz, J.C. Extended Habit Training Reduces Dopamine Mediation of Appetitive Response Expression, Journal of Neuroscience 25, 6729-33, 2005.
While dopamine plays a key role in the performance of goal-directed behaviors, this role diminishes as the behavior has become an overlearned 'habit'.

Davidson, M.C., Horvitz, J.C. Tottenham, N. , Fossella, J.A. Watts, R. Ulug, A.M. and Casey, B.J. Differential cingulate and caudate activation following unexpected nonrewarding stimuli, Neuroimage, 23, 3, 1039-45, 2004.
Neuronal activity in two dopamine target regions in humans, the dorsal striatum and anterior cingulate, is modulated by violations in the expectations of rewarding and non-rewarding events.

Eyny, Y.S. and Horvitz, J.C. Opposing roles of D1 and D2 receptors in appetitive conditioning. Journal of Neuroscience, 23, 1584-87, 2003.
There are two 'families' of dopamine receptors, D1 and D2. These receptors have different structures, and dopamine binding to D1 versus D2 receptors produce different, and in some cases opposite, effects on intracellular events (such as cyclic AMP production). This paper shows that D1 and D2 receptors also play opposing roles in simple associative learning.

Horvitz, J.C. Dopamine, Parkinson’s disease, and volition. Commentary in Behavioral and Brain Sciences, 25(5), 586, 2002.

Horvitz, J.C. Dopamine gating of glutamatergic sensorimotor and incentive motivational inputs to the striatum. Behavioral Brain Research, 137, 65-74, 2002.
This paper suggests a means by which dopamine's motivational and sensori-motor functions can be viewed as the result of a single fundamental mechanism, i.e., the modulation of glutamate inputs from limbic and sensori-motor brain regions.

Horvitz, J.C. and Eyny, Y.S. (2000) Dopamine D2 receptor blockade reduces response likelihood but does not affect latency to emit a learned sensory-motor response: Implications for parkinson's disease. Behavioral Neuroscience 114 (5), 934-939.
Reductions in brain dopamine activity produce reductions in movement. Interestingly, however, this reduction in movement is seen in some instances and not in others.  Parkinson's patients suffer a loss of most of their nigrostriatal dopamine neurons. There is a well-known anecdote about the Parkinson's patients in the hospital who have great difficulty walking.  However, when a fire breaks out in the hospital, the Parkinson's patients walk quickly out of the building, with little difficulty.  (Patients with spinal cord injuries affecting leg movement could not do so).  Dopamine plays a role in the ability to move, but the nature of its role is unclear.  Dopamine does not seem to be necessary in order to perform a movement in response to a "strong eliciting stimulus" (e.g., the fire alarm).  The paper above shows that rats under the influence of a dopamine receptor-blocking drug will show normal movement when presented with a strong (in this case, well-learned) eliciting stimulus, but will show severe deficits when the same movement requires internal generation.

Horvitz, J.C. (2000) Mesolimbic and nigrostriatal dopamine responses to salient non-reward events. Neuroscience, 96, 651-656 [PDF].
Dopamine neurons of the mesolimbic and nigrostriatal systems respond to reward events.  However, these neurons also respond to other types of salient events.  The midbrain dopamine neurons appear to become activated by events whose salience derives from primary or conditioned reward properties, novelty, aversive properties, conditioned aversive properties, or physical intensity (e.g., loudness, brightness, rapid onset).  The fact that midbrain dopamine neurons are driven by a wide category of salient environmental stimuli suggests that, while dopamine activity plays a role in reinforcement or incentive motivational processes, dopamine neurons do not carry a reward signal to target regions of the brain.

Horvitz, J.C., Sterwart, T., and Jacobs, B.L. (1997)  Burst activity of ventral tegmental dopamine neurons is elicited by sensory stimuli. Brain Research 759, 251-258.
Dopamine neurons in the ventral tegmental area (VTA) are widely believed to provide a reward signal. Indeed, VTA dopamine neurons respond to reward events, like food or conditioned stimuli signallingfood delivery. However, this paper demonstrates that the VTA dopamine neurons also respond to salient sensory events which are not rewarding or reward-related. The paper discusses the possiblity that mesolimbic dopamine activity may play a role in attentional processes, rather than a specific role in reward.

Horvitz, J.C., Richardson, W.B., and Ettenberg, A. (1993) Dopamine receptor blockade and thirst produce differential effects on drinking behavior. Pharmacology Biochemistry and Behavior, 45, 725-728.
Animals under the influence of dopamine antagonist drugs show reduced rates of responding for natural rewards such as food and water. However, it is possible that these reductions in motivated behavior reflect a drug-induced blunting of primary motivational states, hunger and thirst, rather than reducing the reward properties of the reinforcer. This paper demonstrates that dopamine disruptions do not reduce thirst. Reductions in water-reinforced behavior seen in dopamine-disrupted animals (Ettenberg and Horvitz 1990), then, cannot be accounted for by reductions in the animal's primary motivational state.

Horvitz, J.C. and Ettenberg, A. (1991) Conditioned incentive properties of a food-paired conditioned stimulus remain intact during dopamine receptor blockade. Behavioral Neuroscience, 105, 536-541.
Investigators have suggested that dopamine activity may underlie the ability of conditioned incentive stimuli (events that have previously been associated with primary rewards) to activate and motivate behavior. According to this view, dopamine would not mediate the reinforcing effect of cocaine, but instead would underlie the motivational/arousing effects of seeing, say, the cocaine-spoon. This study asked whether stimuli that have previously been paired with reward, maintain their motivational/arousing (conditioned incentive) properties even when brain dopamine activity has been disrupted. The results indicate that while dopamine receptor blockade reduces overall levels of behavioral activity, it does not specifically attenuate the incentive properties of conditioned stimuli previously paired with reward.

Horvitz, J.C. and Ettenberg, A. (1989) Haloperidol blocks the response-reinstating effects of food reward: A methodology for separating neuroleptic effects on reinforcement and motor processes. Pharmacology, Biochemistry, and Behavior 31, 861-865.
This study introduces a methodology for asking whether dopamine antagonists disrupt reinforcement, without the motor confound that usually clouds such investigations. Rats run down a straight-arm alley to receive food reward in a goal box. After animals have acquired the running behavior, they undergo a series of extinction trials until running speeds have slowed again. Nondrugged animals will show a reinstatement of the running behavior after a single additional reinforcement trial. This response-reinstatement is blocked in animals under the influence of a dopamine antagonist drug during the reinforced trial. Since all animals are tested 24 hr after the reinforcment/drug trial, drug-induced motor artifacts cannot account for these results. The results provide evidence for a dopamine involvement in food reinforcement. The methodology introduced in this paper has subsequently been employed to investigate dopamine involvement in the reinforcing properties of water, cocaine, amphetamine, and heroin.