Participants
A total of 16 right-handed volunteers (age range 18-45 years old; 8 men, mean age 29.25 years, SD = 4.41 years and 8 women, mean age 27.38 years, SD = 5.12 years) were recruited via advertisement and from an existing subject pool. They had no background of mental disorder (self-reported). Handedness was measured by actual manual performance (self-reported). The local research ethics committee approved the study procedures. All participants signed a consent form after the study procedures had been explained to them, and received £10 for their participation.
Apparatus and materials
The film set (the same as used in the previous studies in our laboratory [15–17]) consisted of nine clips, separated by blank intervals (dark blue screen) 10-25 s long. The first three clips were used only to familiarise participants with the experimental procedure. The last six clips, used to induce emotions under experimental conditions, were presented in two blocks in the order N (neutral), P (pleasant), U (unpleasant), N, U, P. Each film clip lasted about 2 min. The set, shown using a Sharp video recorder (VC-A30HM) connected to a 20-inch Sharp colour TV monitor (DV-5101 A), was viewed from a distance of 2 m.(The supplier: Argos, London, UK).
The acoustic startle stimuli (consisting of a 50-ms presentation of a 92.5 dB (A) burst of white noise, with quasi-instantaneous rise time) were superimposed on the soundtracks (ranging from 40 to 60 dB) of the film clips, at moments of relatively low sound level, and presented monaurally via headphones (Telephonics TDH-39P) (The supplier: Argos, London, UK). During each clip, 3 startle stimuli were presented (total = 27). To increase unpredictability, they were presented with varying interstimulus intervals of 20 to 90 s after clip onset. The responses to the last 18 acoustic startle stimuli (during the last 6 clips) were included in the analyses, excluding the responses to the first 9 acoustic stimuli (during the first 3 clips, which were only for habituation).
To record electromyographic (EMG) activity of the orbicular oculi muscle, two 6 mm disc electrodes (Ag/AgCl) filled with electrolyte paste (SLE, Croydon, UK) were placed approximately 1 cm below the middle of the lower eyelid and 1 cm below the outer corner of the right eye, so that the second electrode was about 1 cm lateral and slightly higher than the first but both were parallel to the lower rim of the eyelid. An additional ground electrode was placed behind the right ear over the mastoid. Raw EMG signals were recorded, amplified, filtered, stored and analysed by a computerised startle response monitoring system (SR Instruments, San Diego, CA, USA). The analytic program treats the first 20 ms after presentation of each startle stimulus as a baseline for that trial. It then calculates latency (ms) to startle onset and peak EMG amplitude (in arbitrary analogue-to-digital units; 1 unit equals 1.2 μV, SR-Lab Program) over the 95 ms following startle onset. Trials with an unstable baseline (shift >20 units) were eliminated. Samples were taken at 1 ms (1 KHz sampling rate). The lower band pass alternative provided by the apparatus (0-500 Hz) was used throughout. The scoring criteria were identical to those used in previous studies from our laboratory [14–19]. Trials were rejected if there was evidence of excessive activity (including a premature eyeblink) during the baseline period. They were also rejected if there was no evidence of an eyeblink having been evoked by the startle probe. Altogether, 16.35% of trials were excluded on one or other of these criteria.
The affective content of each clip was rated as each clip ended (during the blank interval) on a single 11-point (-5 to +5) scale, from extremely unpleasant (for example, depressed, disgusted, angry, anxious; scored as -5), through neutral (scored as 0) to extremely pleasant (for example, happy, relaxed; scored as +5).
Experimental design and procedure
The study consisted of two consecutive sessions, on a single occasion. The acoustic stimuli were presented monaurally to one of the ears in each session.
Participants (counterbalanced for sex) were randomly assigned in equal numbers to one of the two ear orders (left ear (session 1) - right ear (session 2); right ear (session 1) - left ear (session 2)), so that eight participants (four men and four women) received acoustic probes as well as the soundtrack of the film clips, first to the right and then to the left ear; the remaining eight participants (four men and four women) received acoustic probes first to the left and then to the right ear.
Participants were told in advance that they would be tested twice, once with left and once with right ear stimulation, while viewing a series of film clips with either pleasant, unpleasant or neutral content; that each sequence should be watched as long as it was on screen; and that throughout the experiment they would hear occasional bursts of noise through the headphones that would be neither painful nor harmful and should be ignored. The electrodes and headphones were then attached and participants were asked to keep a comfortable position in the chair while watching the video, avoiding gross body movements, and to relax, concentrate and not to attempt to control their emotions, whether positive or negative. An experimenter was present throughout the session. During each session, the affective content of each clip was rated as each clip ended (during the blank interval).
Data reduction and analysis
The data on each of the dependent measures (affective ratings, response amplitude and latency to response onset) were separately analysed by a three-way (valence (pleasant, neutral, and unpleasant) × ear side (left and right) × ear order (left to right and right to left)) multivariate analysis of variance (MANOVA; Wilk's F), with valence and ear side as within-subjects variables and ear order as a between-subjects factor. As there were no main or interaction effects of ear order, this variable was excluded from all further analyses and the data were subjected separately to a three-way MANOVA (sex (men and women) × ear side (left and right) × valence (pleasant, neutral, and unpleasant)), with ear and valence as within-subjects variables and sex as a between-subjects factor. Since no significant main or interaction effects were found for the measures of baseline EMG and latency to response onset, only the findings on affective ratings and startle amplitude are reported here.
Although no significant interaction effect appeared in the above analysis, in order to compare the present data with that reported previously with slides ([9]; a linear trend of valence effect separately for each ear), the ear side variable was dropped from further analyses and the data for each ear separately were subjected to a two-way MANOVA (sex (men and women) × valence (pleasant, neutral, and unpleasant)), with valence as a within-subjects variable and sex as a between-subjects factor, followed by polynomial contrast tests (assessed by t) on valence effects.