Fear-Potentiated Startle

views updated

Fear-Potentiated Startle

When a stimulus such as a light, which engenders little behavioral effect before pairing, is paired with an aversive stimulus such as a foot shock, the light (conditioned stimulus) can elicit seemingly fearful responses in animals: autonomic changes, freezing, and an increase in the amplitude of the startle reflex elicited by an auditory stimulus in the presence of the light. The last is called the fear-potentiated startle effect and can occur with an auditory, visual, tactile, or olfactory conditioned stimulus under conditions where startle is elicited by either a loud sound or an air puff.

Fear-potentiated startle is a valid measure of classical conditioning because it occurs only following paired rather than unpaired or "random" presentations of the conditioned stimulus. Potentiated startle shows considerable temporal specificity because its magnitude in testing is greatest at the interval after light onset that matches the light-shock interval in training. This paradigm offers a number of advantages as an alternative to most animal tests of fear or anxiety because it involves no operant and is reflected by an enhancement rather than a suppression of continuing behavior. Drugs like clonidine, morphine, diazepam, and buspirone, which differ in their mechanism of action yet all reduce fear or anxiety in humans, decrease potentiated startle in rats. Conversely, drugs like yohimbine, piperoxane, and B-carbolines, which induce anxiety in normal people and exaggerate it in anxious people, increase the magnitude of fear-potentiated startle in rats.

Neural Systems Involved in Fear-Potentiated Startle

A major advantage of the fear-potentiated startle paradigm is that fear is measured by a change in a simple reflex. Hence, with potentiated startle, fear is expressed through some neural pathway(s) activated by the conditioned stimulus that connects to the startle pathway. Figure 1 shows a schematic summary diagram of the neural pathways we believe are required for fear-potentiated startle, given a visual conditioned stimulus and foot shock as the unconditioned stimulus.

The Acoustic Startle Pathway

In the rat, the latency of acoustic startle is six milliseconds, recorded electromyographically in the foreleg, and eight milliseconds in the hind leg. This very short latency indicates that only a few synapses can be involved in mediating acoustic startle. Using a variety of techniques, we showed that acoustic startle was mediated by a pathway that includes auditory neurons embedded in the auditory nerve called cochlear root neurons, an area just dorsal to the superior olives in the nucleus reticularis pontis caudalis, and motoneurons in the spinal cord. Bilateral chemical lesions of these cell groups eliminate startle, whereas lesions in a variety of other auditory or motor areas do not. Startlelike responses can be elicited electrically from each of these nuclei, with progressively shorter latencies as the electrode is moved down the pathway.

Where Does Fear Activate the Startle Pathway?

By eliciting startlelike responses electrically from various points along the startle pathway in the presence and absence of a light previously paired with a shock, we concluded that fear ultimately alters transmission at the nucleus reticularis pontis caudalis. Injection of retrograde tracers into this part of the startle pathway indicated that it receives direct projections from the central nucleus of the amygdala, an area of the brain long implicated in fear, as well as from an area in the mesecenphalic reticular formation and deep layers of the superior colliculus.

Lesions of the Amygdala Block Fear-Potentiated Startle

Chemical lesions of either the basolateral or central nucleus of the amygdala following fear conditioning completely eliminate potentiated startle. In contrast, lesions of a variety of other brain areas, including the frontal cortex, insular cortex, visual cortex, hippocampus, septal nuclei, superior colliculus, red nucleus, and cerebellum, do not. Low-level electrical stimulation of the amygdala markedly increases acoustic startle amplitude at stimulus currents and durations that do not produce any other signs of behavioral activation.

Role of Different Amygdala Efferent Projections in Fear-Potentiated Startle

The pathway between the central nucleus of the amygdala and the part of the nucleus reticularis pontis caudalis that is critical for startle involves the caudal division of the ventral amygdalofugal pathway, which also sends collaterals to many brain-stem target areas involved in the somatic and autonomic symptoms of fear and anxiety. Lesions along this pathway completely block potentiated startle. In contrast, lesions of other major projections from the central nucleus of the amygdala do not. In addition to this direct pathway, an indirect pathway between the central nucleus of the amygdala and the deep superior colliculus/mesecephalic reticular formation is required for fear-potentiated startle because inactivation of this region completely blocked expression but not acquisition of fear-potentiated startle using a visual conditioned stimulus.

Convergence of Light and Shock Input at the Amygdala

Figure 1 shows that visual information reaches the amygdala by way of two parallel pathways. One involves direct retinal inputs to the lateral posterior nucleus of the thalamus, which projects directly to the basolateral amygdala and perirhinal cortex. The other involves retinal inputs to the dorsal lateral geniculate nucleus, which projects indirectly to the perirhinal cortex via the visual cortex. Lesions of both of these visual thalamic nuclei together, but not either one alone, blocked fear-potentiated startle when a visual, but not an olfactory, conditioned stimulus was used. When an auditory conditioned stimulus is used, this involves parallel inputs from the auditory thalamus to the perirhinal cortex either directly or indirectly via the auditory cortex. Pretraining or post-training lesions of the entire auditory thalamus completely blocked fear-potentiated startle to an auditory but not to a visual conditioned stimulus. Post-training lesions restricted to the main body of the medial geniculate, which projects to the perirhinal cortex via auditory cortex, also specifically blocked fear-potentiated startle to the auditory CS. Pain information reaches the amygdala via parallel pathways that include the caudal granular/dysgranular insular cortex and the posterior intralaminar nuclei of the thalamus. Pretraining lesions of both insular cortex and the posterior intralaminar nuclei of the thalamus, but not lesions of either structure alone, blocked the acquisition of fear-potentiated startle. However, post-training combined lesions of these areas together did not prevent expression of conditioned fear. These results suggest that parallel cortical and subcortical pathways are involved in relaying shock information during fear conditioning.

Role of the Anterior Perirhinal and Insular Cortex in Fear-Potentiated Startle

The perirhinal cortex, which receives either visual or auditory conditioned stimulus information, projects directly to the lateral and basolateral amygdala. Posttraining lesions of the anterior perirhinal cortex completely blocked the expression of fear-potentiated startle when a visual conditioned stimulus is used, provided the lesion destroyed both the dygranular and agranular portions of the perirhinal cortex. Posttraining lesions of the perirhinal area (including secondary auditory cortices) blocked fear-potentiated startle to both an auditory and visual conditioned stimulus. However, reliable potentiated startle was observed after retraining in animals sustaining main geniculate body lesions (which would destroy cortical connections between the thalamus and perirhinal cortex) or following pretraining lesions of the perirhinal area. These data suggest that cortical areas normally are used for the expression of fear conditioning but that subcortical areas can take over if the cortex is damaged. Finally, as mentioned before, shock information seems to require the insular cortex, which in turn projects directly to the lateral nucleus of the amygdala.

Glutamate Receptors in the Amygdala Are Critical for Fear-Potentiated Startle

Because conditioned and unconditioned stimulus information converge at the lateral and basolateral amygdala nuclei, this could be the site of plasticity for fear-potentiated startle. In fact, local infusion of NMDA antagonists into the amygdala blocked the acquisition but not the expression of fear-potentiated startle when a visual, auditory, or olfactory conditioned stimulus was used. This blockade of fear acquisition probably was not the result of preventing shock information from getting to the amygdala because this treatment also blocked the acquisition of second order fear-potentiated startle, which does not involve shock during second order training. Following conditioning, local infusion of non-NMDA antagonists block the expression of fear-potentiated startle.

Fear-Potentiated Startle in Humans

Fear-potentiated startle also can be measured in humans. In one test people are told that when a certain colored light comes on, they might get a shock on the wrist, whereas when a different-colored light comes on, they will not get a shock. Startle is elicited with bursts of noise through earphones, and the eye-blink component of startle is measured electromyo-graphically from the obicularis oculi muscles. Startle amplitude was consistently higher in the presence of the light that signals shock. The size of this increase depended on when the subject expected the shock based on verbal instructions; the increase was also evident using conditioning procedures. Brain imaging studies show that the amygdala is activated during this verbally mediated fear-potentiated startle test and that patients with lesions of the amygdala failed to display fear-potentiated startle. People also show an increase in startle when they see scary pictures, such as a gun in the face, a dog about to bite, or mutilated bodies. The size of the increase in startle was directly related to the subjects' degree of negative valence and arousal.

Conclusion

The fear-potentiated startle paradigm has been useful for elucidating neural substrates of fear and anxiety. Conditioned fear appears to result when a formerly neutral stimulus comes to activate the amygdala after being paired with an aversive stimulus. Activation of the central nucleus of the amygdala increases startle via direct and indirect connections between the amygdala and a specific point in the acoustic startle pathway. More generally, the central nucleus of the amygdala and its efferent projections to the brain stem may constitute a central fear system that produces a constellation of fearlike behaviors in animals and people. Finally, the acquisition of conditioned fear may involve an NMDA-dependent process at the level of the amygdala.

Bibliography

Berg, W. K., and Davis, M. (1985). Associative learning modifies startle reflexes at the lateral lemniscus. Behavioral Neuroscience 99, 191-199.

Brown, J. S., Kalish, H. I., and Farber, I. E. (1951). Conditional fear as revealed by magnitude of startle response to an auditory stimulus. Journal of Experimental Psychology 41, 317-328.

Campeau, S., and Davis, M. (1995a). Involvement of subcortical and cortical afferents to the lateral nucleus of the amygdala in fear conditioning measured with fear-potentiated startle in rats trained concurrently with auditory and visual conditioned stimuli. Journal of Neuroscience 15, 2,312-2,327.

—— (1995b). Involvement of the central nucleus and basolateral complex of the amygdala in fear conditioning measured with fear-potentiated startle in rats trained concurrently with auditory and visual conditioned stimuli. Journal of Neuroscience 15, 2,301-2,311.

Davis, M. (2000). The role of the amygdala in conditioned and unconditioned fear and anxiety. In J. P. Aggleton, ed., The amygdala, Vol. 2. Oxford, UK: Oxford University Press.

Davis, M., and Astrachan, D. I. (1978). Conditioned fear and startle magnitude: Effects of different footshock or backshock intensities used in training. Journal of Experimental Psychology: Animal Behavior Processes 4, 95-103.

Davis, M., Falls, W. A., Campeau, S., and Kim, M. (1993). Fear-potentiated startle: A neural and pharmacological analysis. Behavior Brain Research 58, 175-198.

Davis, M., Gendelman, D. S., Tischler, M. D., and Gendelman, P. M. (1982). A primary acoustic startle circuit: Lesion and stimulation studies. Journal of Neuroscience 6, 791-805.

Davis, M., Schlesinger, L. S., and Sorenson, C. A. (1989). Temporal specificity of fear-conditioning: Effects of different conditioned stimulus-unconditioned stimulus intervals on the fear-potentiated startle effect. Journal of Experimental Psychology: Animal Behavior Processes 15, 295-310.

Falls, W. A., and Davis, M. (1994). Fear-potentiated startle using three conditioned stimulus modalities. Animal Learning and Behavior 22, 379-383.

Funayama, E. S., Grillon, C., Davis, M., and Phelps, E. A. (2001). A double dissociation in the affective modulation of startle in humans: Effects of unilateral temporal lobectomy. Journal of Cognitive Neuroscience 13, 721-729.

Gewirtz, J., and Davis, M. (1997). Second order fear conditioning prevented by blocking NMDA receptors in the amygdala. Nature 388, 471-474.

Grillon, C., Amelia, R., Merikangas, K., Woods, S. W., and Davis, M. (1993). Measuring the time course of anticipatory anxiety using the fear-potentiated startle reflex. Psychophysiology 30, 340-346.

Grillon, C., and Davis, M. (1997). Fear-potentiated startle conditioning in humans: Effects of explicit and contextual cue conditioning following paired versus unpaired training. Psychophysiology 34, 451-458.

Lang, P. J., Bradley, M. M., and Cuthbert, B. N. (1990). Emotion, attention, and the startle reflex. Psychology Review 97, 377-395.

Lee, Y., Lopez, D. E., Meloni, E. G., and Davis, M. (1996). A primary acoustic startle circuit: Obligatory role of cochlear root neurons and the nucleus reticularis pontis caudalis. Journal of Neuroscience 16, 3,775-3,789.

Meloni, E. G., and Davis, M. (1999). Muscimol in the deep layers of the superior colliculus/mesencephalic reticular formation blocks expression but not acquisition of fear-potentiated startle in rats. Behavioral Neuroscience 113, 1,152-1,160.

Paschall, G. Y., and Davis, M. (2002). Olfactory mediated fear potentiated startle. Behavioral Neuroscience 116, 4-12.

Phelps, E. A., O'Connor, K. J., Gatenby, J. C., Gore, J. C., Grillon, C., and Davis, M. (2001). Activation of the left amygdala to a cognitive representation of fear. Natural Neuroscience 4, 437-441.

Rosen, J. B., and Davis, M. (1988). Enhancement of acoustic startle by electrical stimulation of the amygdala. Behavioral Neuroscience 102, 195-202.

Rosen, J. B., Hitchcock, J. M., Miserendino, M. J. D., Falls, W. A., Campeau, S., and Davis, M. (1992). Lesions of the perirhinal cortex but not of the frontal, medial prefrontal, visual, or insular cortex block fear-potentiated startle using a visual conditioned stimulus. Journal of Neuroscience 12, 4,624-4,633.

Sananes, C. B., and Davis, M. (1992). N-Methyl-D-Aspartate lesions of the lateral and basolateral nuclei of the amygdala block fear-potentiated startle and shock sensitization of startle. Behavioral Neuroscience 106, 72-80.

Shi, C. J., and Davis, M. (1999). Pain pathways involved in fear conditioning measured with fear-potentiated startle: Lesion studies. Journal of Neuroscience 19, 420-430.

—— (2001). Visual pathways involved in fear conditioning measured with fear-potentiated startle: Behavior and anatomic studies. Journal of Neuroscience 21, 9,844-9,855.

Walker, D. L., and Davis, M. (2002). The role of glutamate receptors within the amygdala in fear learning, fear-potentiated startle, and extinction. Pharmacology, Biochemistry, and Behavior 71, 379-392.

MichaelDavis

Learning and Memory