From the Wikipedia page 
The Isthmian script is a very early Mesoamerican writing system in use in the area of the Isthmus of Tehuantepec from perhaps 500 BCE to 500 CE, although there is disagreement on these dates. It is also called the La Mojarra script and the Epi-Olmec script ('post-Olmec script').
Isthmian script is structurally similar to the later Maya script, and like Maya uses one set of characters to represent logograms (or word units) and a second set to represent syllables.
The four most extensive Isthmian texts are those found on:
Other texts include:
- A few Isthmian glyphs on four badly weathered stelae — 5, 6, 8, and probably 15 — at Cerro de las Mesas.
- Approximately 23 glyphs on the O'Boyle "mask", a clay artifact in a private collection of unknown provenance.
- A small number of glyphs on a pottery-sherd from Chiapa de Corzo. This sherd has been assigned the oldest date of any Isthmian script artifact: 450-300 BCE.
In a 1993 paper, John Justeson and Terrence Kaufman proposed a partial decipherment of the Isthmian text found on the La Mojarra Stela, claiming that the language represented was a member of the Zoquean language family. (Justeson and Kaufman (1993)). In 1997, the same two epigraphers published a second paper on Epi-Olmec writing, in which they further claimed that a newly discovered text-section from the stela had yielded readily to the decipherment-system that they had established earlier for the longer section of text. This led to a Guggenheim Fellowship for their work, in 2003.
The following year, however, their interpretation of the La Mojarra text was disputed by Stephen D. Houston and Michael D. Coe, who had tried unsuccessfully to apply the Justeson-Kaufman decipherment-system to the Isthmian text on the back of the hitherto unknown Teotihuacan-style mask (which is of unknown provenance and is now in a private collection). (Brigham Young University press-release). Despite the lack of provenance, Houston "is confident it [the mask text] was written sometime between A.D. 300 and 500" which would place it 150 to 250 years later than the La Mojarra stela.
The matter is still under discussion. In Lost Languages (2008) W. Andrew Robinson summarises the position as follows:
'Overall, then, the case for the Justeson/Kaufman 'decipherment' of Isthmian is decidedly unproven and currently rests on shaky foundations ... What it needs, more urgently than some other 'decipherments' given its evident linguistic sophistication, is the discovery of a new text or texts as substantial as the one found at La Mojarra in 1986.