The temporal course of startle reflex modulation and autonomic response patterns to fear-relevant and fear-irrelevant pictures in subjects with high and low levels of animal fear was investigated. Thirty-eight high-fear and 48 low-fear volunteers viewed photos of snakes and spiders and pictures of neutral and pleasant content. The slides were presented for 6 s or for only 150 ms, depending on the group. Acoustic startle probes were presented at five different times after slide onset. Relative potentiation of the startle responses started 300 ms after onset of snake/spider pictures in fearful subjects. This fear-potentiated startle effect was maintained for the later probe times and was identical in the 150-ms condition. Fear-relevant pictures also prompted a sympathetically dominated autonomic response profile in fearful persons. These data support the idea that fear can be activated very rapidly, requiring only minimal stimulus input.

A basic feature of human evaluative conditioning is that the reported acquired valence of a previously neutral conditioned stimulus (CS) that has been paired with a valenced unconditioned stimulus (US), is resistant to extinction. The present study investigated whether startle modulation, sometimes presented as an index of acquired valence, reflected this basic feature. In a differential fear conditioning paradigm (n = 38) with an electrocutaneous stimulus as the US, a strong extinction manipulation was conducted by removing the US-electrodes and by extended extinction trials. At the end of extinction, the results corroborated previous findings of evaluative conditioning in that the reported valence for CS+ was still more negative than for CS−, despite disappearance of the differential skin conductance responses. However, startle modulation did not show resistance-to-extinction. Results were discussed in terms of recent conceptualizations of extinction.