The midbrain is an essential part of the brain for the homeostasis of our organism and for our survival.

Inside it we can locate the pretectum, an area involved in unconscious visual processing and related to automatic processes such as the oculomotor reflex or REM sleep.

In this article we explain what the pretectum is, where it is located and how this brain region is structured . In addition, we explain the main functions it performs, and an example of one of the most common disorders following injury to this area of the brain.

Pretectum: definition, location and structure

The pretectal area or pretectum is a region of the brain located in the midbrain, a structure that connects the brain stem to the diencephalon. This area is part of the subcortical visual system and has reciprocal connections with the retina. It is composed of several highly interconnected nuclei.

In the midbrain is located the tectum, a structure located at the back of the brain, composed of two upper and two lower collicles. The pretectum is located in the anterior part of the upper colicle and posterior to the thalamus (centre of relay of the sensory information that goes to the cerebral cortex), and over the periaqueductal grey substance and the nucleus of the posterior commissure.

Although they have not been clearly delimited, the seven nuclei of the pretectum receive each one of them a name with its respective region ; the five primary nuclei are: the olive pretectal nucleus, the nucleus of the optical tract, the anterior nucleus, the medial nucleus and the posterior nucleus. Two additional nuclei have been identified: the pretectal commissural area and the posterior limitans.

Although these last two nuclei have not been studied in the same way as the five primary nuclei, research has shown that the two additional nuclei receive connections from the retina, suggesting that they would also play a role in processing visual information. To clarify this, we will now look at the functions performed by the pre-tectal nuclei.

Functions

The pretectum is part of the subcortical visual system and the neurons of this structure respond to variable intensities of illumination . The cells of the pretectal nuclei are mainly involved in mediating unconscious behavioural responses to acute changes in light.

In general, these responses include the initiation of certain optokinetic reflexes, although, as we will see below, the pretectum also participates in other processes such as the regulation of nociception (the coding and processing of potentially harmful or painful stimuli) or REM sleep.

1. The photomotor reflex

The photomotor or pupillary light reflex is produced when the pupil of the eye responds to light stimuli , increasing or decreasing its diameter. This reflex is mediated by several of the pretectum nuclei, in particular the pretectal olive nucleus, which receive information from the light level of the ipsilateral retina through the optical tract.

The pretectal nuclei gradually increase their activation in response to increasing levels of illumination, and this information is transmitted directly to the Edinger-Westphal nucleus, which is responsible for transmitting the nerve impulses and sending the signal to the pupillary sphincter, through the ciliary ganglion, so that pupillary contraction occurs.

2. Tracking eye movements

The pretectal nuclei, and in particular the nucleus of the optical tract, are involved in coordinating eye movements during slow eye tracking. These movements allow the eye to closely follow a moving object and catch it after an unexpected change in direction or speed.

Retinal neurons that are direction sensitive and located in the nucleus of the optical tract , provide information of horizontal displacement errors in the retina through the lower olive. In daylight, this information is detected and transmitted by neurons with large receptive fields, while parafovial neurons with small receptive fields do so in darkness or twilight.

It is in this way that the nucleus of the optical tract is able to send the information of the errors in the retina to guide the eye movements. In addition to its role in maintaining such eye movements, the pretectum is activated during optokinetic nystagmus in which the eye returns to a central, forward-facing position after an object being followed moves out of the field of vision.

3. Antinociception

The anterior pretectal nucleus participates in the active reduction of the perception of painful stimuli or antinociception . Although the mechanism by which the pretectum alters the body’s response to these stimuli is still unknown, research suggests that the activity of the ventral pretectal anterior nucleus includes cholinergic and serotonergic neurons.

These neurons activate downstream pathways that make synapses in the spinal cord and inhibit nociceptive cells in its dorsal horn. In addition to its direct antinociceptive mechanism, the anterior pretectal nucleus sends projections to regions of the brain that, through connections in the somatosensory cortex, regulate pain perception. Two of these regions known to project the pretectum are the uncertain zone (a subthalamic nucleus) and the posterior nucleus of the thalamus.

Several studies have found that the pretectal anterior dorsal nucleus decreases most significantly the perception of brief pain, while its ventral part would do so in cases of chronic pain. Because of its role in reducing chronic pain, it has been suggested that abnormal activity of this pretectal nucleus may be involved in central neuropathic pain syndrome.

4. REM sleep

Regarding rapid eye movement sleep or REM sleep, research suggests that multiple pretective nuclei may be involved in the regulation of this type of sleep and other similar behaviours. It has been suggested that the pretectum, along with the superior colliculus, may be responsible for causing non-circadian disturbances in REM sleep behaviors.

Animal studies, specifically in albino rats, have shown that the pretectal nuclei that receive information from the retina, particularly the nucleus of the optical tract and the posterior pretectal nucleus, are partly responsible for initiating rapid eye movement sleep.

The discovery of the existence of projections from the pretectum to several thalamic nuclei involved in cortical activation during REM sleep, specifically to the supraquiasmatic nucleus which is part of a regulatory mechanism of this type of sleep, would support this last hypothesis.

Pre-tectal syndrome

Pretectal syndrome, also called Parinaud syndrome or mesencephalic dorsal syndrome, refers to a set of clinical signs and symptoms that include alterations in ocular motility and whose main cause is the involvement of the pretectum and other adjacent brain areas.

This syndrome causes alterations in the ocular motility, both externally and internally. The most common signs are the following:

  • Pupillary alterations : asymmetry of pupil size, dissociated reaction to light, accommodative paresis and pupillary areflexia

  • Paralysis of the vertical gaze upwards (supranuclear type).

  • Eyelid retraction (Collier’s sign).

  • Lid lag sign : when the upper eyelid cannot maintain its relative position to the eyeball (when moving the eyes downwards).

  • Convergence-retraction nystagmus : when the person tries to look up, the eyes return to their central position and the eyeballs retract.

Bibliographic references:

  • Gamlin, P. D. (2006). The pretectum: connections and oculomotor-related roles. Progress in brain research, 151, 379-405.

  • Keane, J. R. (1990). The pretectal syndrome: 206 patients. Neurology, 40(4), 684-684.

  • Miller, A. M., Miller, R. B., Obermeyer, W. H., Behan, M., & Benca, R. M. (1999). The pretectum mediates rapid eye movement sleep regulation by light. Behavioral neuroscience, 113(4), 755.