An anatomical substrate for the spatiotemporal transformation

Abstract

The purpose of the present experiments was to test the hypothesis that the metrics of saccades caused by the activation of distinct collicular sites depend on the strength of their projections onto the burst generators. This study of morphofunctional correlations was limited to the horizontal components of saccades. We evoked saccades by stimulation of the deeper layers of the superior colliculus (SC) in alert, head-fixed cats. We used standard stimulus trains of 350 msec duration, 200 Hz pulse rate, and intensity set at two times saccade threshold in all experiments. Evoked saccades were analyzed quantitatively to determine the amplitude of the horizontal component of their "characteristic vectors". This parameter is independent of eye position and was used as the physiological, saccade-related metric of the stimulation sites. Anatomical connections arising from these sites were visualized after anterograde transport of biocytin injected through a micropipette adjoining the stimulation electrode. The stimulation and injection sites were, therefore, practically identical. We counted boutons deployed in regions of the paramedian pontine reticular formation reported to contain long-lead and medium-lead burst neurons of the horizontal burst generator. Regression analysis of the normalized bouton counts revealed a significant positive correlation with the size of the horizontal component of the characteristic vectors. This data supports a frequent modelling assumption that the spatiotemporal transformation in the saccadic system relies on the graded strength of anatomical projections of distinct SC sites onto the burst generators.

Fig. 1.

Plots of horizontal (ΔH) and vertical (ΔV) size of saccades (ordinate) elicited in response to the electrical stimulation (2 × T, 70 pulses) of the superior colliculus versus initial horizontal (H₁) and vertical (V₁) eye position (abscissa). Solid lines are the linear regression lines through the data (open circles, vertical; solid circles, horizontal) and obey the expressions displayed. The “characteristic vector” can be read off the points at which the linear regression lines intersect thevertical dashed line through zero initial eye position.

Fig. 2.

A–F, Horizontal plots of the outlines of the superior colliculi to illustrate the locations of stimulation and injection sites (large crosses) in different animals. Dots show the distributions of biocytin-labeled cells inside the SC. # Cells, Number of labeled cells recovered in the SC; # Fibers, number of labeled fibers counted at their entry in the pons; C, caudal; L, lateral;M, medial; R, rostral. G, Stimulation–injection sites in different animals superimposed on a retinotopic map of the SC (McIlwain, 1986) embedded within the normalized outlines of each animal’s SC. Orientationarrows apply to A–G.H, Two-dimensional plot of the characteristic vectors (abscissa, vertical; ordinate,horizontal) of saccades evoked from corresponding collicular sites.

Fig. 3.

Photomicrographic illustration of the injection sites in NB3 (A) and NB4 (B) and terminal arborizations in the PPRF of NB3 (C). Scale bars: A,B, 1.25 mm; C, 50 μm.

Fig. 4.

Camera lucida reconstructions of the stimulation–injection sites in the superior colliculus in the frontal plane. Different degrees of shading correspond to areas of intense (black), moderate (hatched), and weak (stippled) tracer deposition. Drawings show maximal extent of these areas, as seen in several (5–8) 75-μm-thick sections centered on the electrode tracks. Note that in cases NB12 and NB14 the ventrolateral extensions of the weakly stained regions correspond to labeled fibers coursing in the intermediate white layer. These extensions were not included in the measurements of the size of diffusely labeled zones. MG, Medial geniculate;NTO, nucleus of the optic tract; NPP, posterior pretectal area; SGI, stratum griseum intermedium; SGP, stratum griseum profundum;SO, stratum opticum; PAG, periaqueductal gray matter.

Fig. 5.

Distribution of labeled axons in the SC after medial (A, NB3) or lateral (B, NB4) injections of biocytin. Superpositions of camera lucida tracings from five adjacent frontal sections at the level of the center of injection sites. Only axonal segments visible at low magnification (40×) have been included. Thick arrows delimit the zone occupied by radial axons passing through the deep gray layer before joining the tectobulbospinal tract. Concentric contours correspond, from inward to outward, to dense, moderate, and weak diffuse staining around the injection tracks (compare with Fig. 4). Numbers (2-7) indicate layers of the SC (superficial gray, optic, intermediate gray, intermediate white, deep gray, and deep white, respectively). Aq, Aqueduct;CG, central gray.

Fig. 6.

Camera lucida drawings of labeled predorsal bundle fibers at their entry in the pons. A, B, Topography of sections used to obtain the number of axons for the normalization of bouton counts in experiments NB3 and NB4, respectively. The sections are 1125 μm (A) and 1275 μm (B) posterior to the caudal pole of the trochlear nucleus. A₁,B₁, Higher power drawings of the predorsal bundle region delimited by dashed lines inA and B, respectively. Short solid segments, Labeled descending axons. Dotted lines, Axon collaterals visible at a magnification of 100×. Axon counts are 107 (A₁) and 162 (B₁). Aq, Aqueduct;BC, brachium conjuctivum; CG, central gray; CI, inferior colliculus; DR, dorsal nucleus of the raphe; MLF, medial longitudinal fasciculus; NCF, nucleus cuneiformis;NCS, nucleus centralis superior; NL, nuclei of the lateral lemniscus; NRPo, nucleus reticularis pontis oralis; NRT, nucleus reticularis tegmenti pontis; PDB, predorsal bundle;PN, pontine nuclei; VT, ventral tegmental nucleus (of Gudden).

Fig. 7.

Composite plots (5 sections each) of biocytin-labeled boutons in coronal sections through the PPRF of NB3 (A), NB12 (B), NB2 (C), and NB4 (D).Dashed lines indicate the areas selected for bouton counts. Thin solid lines delimit the reticular core and other pontine nuclei. Vp, Principal sensory nucleus of the trigeminal nerve; VIIIs, superior vestibular nucleus; BC, brachium conjuctivum; NRT, nucleus reticularis tegmenti pontis; NTB, nucleus of the trapezoid body; PVG, periventricular gray;TB, trapezoid body.

Fig. 8.

Scatter plot of the number of boutons per 100 fibers (ordinate) deployed in the PPRF. Dashed vertical lines separate sections that belong to different animals.Small open circles indicate the number of boutons observed in adjacent individual 75 μm sections, whereas large solid circles indicate the average for the animal indicated. The inset is a plot of the average number of boutons deployed in the PPRF per 100 fibers per section (B; ordinate) from each one of the injection sites versus the size of the horizontal component of the characteristic vector of the saccades evoked from the same site (β_H; abscissa). Error bars indicate the SEM. The solid line is the linear regression line through the data and obeys the equation displayed.