A kinetic study of blinking responses in cats

Abstract

Reflexively evoked and eye-related eyelid responses were recorded using the search coil in a magnetic field technique in alert cats. The downward phase of a blink was a large (up to 21 deg), fast (up to 2000 deg s-1) eyelid displacement in the closing direction, with an almost fixed rise time duration (15-20 ms); its maximum velocity was achieved in ~10 ms. Upward eyelid motion was separated into two phases. The first phase consisted of a fast eyelid displacement, with a short duration (approximately 30 ms) and a maximum velocity up to 900 deg s-1. The second phase had an exponential-like form, lasting for 200-400 ms, and a maximum velocity ranging between 30 and 250 deg s-1. Maximum blink velocity in the downward direction was linearly related to maximum velocity of the first upward phase. The first phase in the upward direction was never observed if the eyelid stayed closed for a long period (> 50 ms) or moved slowly in the closing direction before it started to open. In these two cases, the upswing motion of the blink reflex contained only the exponential-like movement characteristic of the second upward phase, and maximum velocity in the downward direction was not related to that of the eyelid upward displacement. Mean duration of eyelid downward saccades was approximately 130 ms, and their peak velocities ranged between 50 and 440 ms. A physiological model is presented explaining the active and passive forces involved in both reflex and saccadic eyelid responses. A second-order system seems to be appropriate to describe the postulated biomechanical model.

Figure 1. Two different types of air puff-evoked blinks

A, examples of blinks evoked by long-lasting (100 ms) strong (3 kg cm⁻²) air puffs. Blinks were initiated by a fast, downward displacement of the eyelid. The opening of the eyelid was divided in two different phases (I and II). The beginning of the second upward phase (II) was sometimes preceded by a conspicuous oscillation (dashed arrow). B, examples of blinks with a slow velocity or a long duration of the downward phase. In these cases, eyelid opening presented only the second upward phase (II). Note that while in A the change from the downward to the upward direction occurred in a short period of time (≈5 ms), in B there was a larger temporal separation between them (> 50 ms). Eyelid profiles were grouped to a common start to show the variability in amplitude of the evoked responses.

Figure 6. Proposed mechanism for downward saccades and blinks

The dotted lines represent the main structures involved in the movement. For a blink, the active contribution of orbicularis oculi motoneurones (OO Mn) is needed. Abbreviations: APO, aponeurosis of levator palpebrae superioris muscle; ME, margins of the upper eyelid; LPS, levator palpebrae superioris muscle; OO, orbicularis oculi muscle; WL, Whitnall's ligament.

Figure 2. Representation of the two different phases in the upward movement of the eyelid, during a blink

A, a blink reflex evoked by a long, strong air puff (100 ms, 3 kg cm⁻²). The upper trace represents eyelid position, and the lower one eyelid velocity. Note that the upward eyelid movement is separated into a first rapid linear phase (I) followed by an exponential displacement (II) by the occurrence of two downward sags. The histogram at the bottom shows the latency to peak velocity for both phases (11.6 ± 2 and 98.2 ± 37 ms, respectively). B and C, profiles of eyelid velocity for the two components of upward eyelid movement: B shows the initial fast upward phase (I in A), and C the late phase of the upward movement (II in A). The linear (B) and S-shaped (C) traces represent the average (n = 5) eyelid position, while the parabolic-like profiles depict eyelid velocity. Data in C were collected from blinks in which the eyelid stayed closed for a long period, or closed slowly (see Fig. 1B), that is, without a noticeable initial upward component. The inset in C shows the average of 10 such blinks. Calibrations of the bars for the inset in C are 200 ms and 5 deg.

Figure 3. Analysis of the relationship between eyelid position and velocity during the two upward phases of reflexively evoked blinks

A, recordings of eyelid position and velocity during air puff-evoked blinks. Parameters taken to build the relationships shown in B and C are indicated: I, first phase in the upswing of the blink; II, second phase in the upswing. B, relationship between maximum velocity (in deg s⁻¹) of downward and upward eyelid responses for the first (I in A) and second (II in A) upward phases. C, relationship between maximum eyelid velocity (in deg s⁻¹) in the upward direction and blink amplitude (in deg) for both first (I) and second (II) upward phases. Linear equations corresponding to each regression line are indicated.

Figure 4. Measurement of the contribution of phases I and II to eyelid upward displacement during a blink and analysis of eyelid movement in the plane of phase

A, five examples from which were obtained the difference in the eyelid displacement for phases I and II of the upward motion. The inset shows an example of how the represented curves were calculated. The continuous line in the inset (arrow) represents the difference between the upward phases of two blinks of equal downward amplitude (dashed lines). B, representation in the plane of phase (position against velocity) for three normal blinks. C, a representation similar to B but for blinks with a low downward velocity or after which the eyelid stayed closed for a time period > 50 ms.

Figure 5. Differences in kinetic properties of downward saccades and reflexively evoked blinks

A, eyelid displacement during a downward saccade and a blink. In order to make the comparison, the two movements have been set to a common start and are of the same amplitude. The dashed arrow indicates the maximum difference in eyelid position between the two movements in the downward direction. B, velocity profiles for both types of movement illustrated in A. Note the different values for maximum velocities and latencies to peak velocity (arrows).